Alrighty~ Some of it may be tricky, because I'm not sure which species are available in the UK and their needs can be slightly different. ^u^;
Enclosure: 10gal tank minimum (for small crabs) with glass lid (with some holes for air exchange) to help hold in heat and humidity. They can grow fast though and by the time they get big, they'll need a 50gal minimum. I went from a 10gal, to a 20gal long, to a 29gal, to a 75gal all in the course of 2 years.
*The tiny kritter keeper/pet keeper "hermit crab kits" are too small even for the smallest of crabs, despite being marketed for hermit crabs.
Substrate: Playsand mixed with eco earth coconut fiber, moistened with salt water to sand castle consistency. The salt water helps prevent mold. Most people mix it to a 5:1 ratio of sand : EE. The depth of the substrate has to be at least 6in, or three times deeper than the biggest crab is tall in order for proper molting. Some species need at least 10in because some dig and tunnel more than others. Molting varies depending on size, but can take 1-4 months for small guys and 5-8+ months for the big ones. So they're definitely a pet you're gonna go without seeing for awhile sometimes. All of mine have been down molting for about a month so far, right now.
*Calcium sand, colored sand, "hermit crab sand" - its all the same thing, and since it clumps and gets hard when it gets moist, its absolutely not suitable to use, despite being marketed for hermit crabs.
Temperature: 80 degrees F, no less than 75 degrees F; anything below that will cause lethargy and slow death because they're cold blooded and require hot temps. Some species are better with 85 degree F - 90 degrees F temps. Most people use undertank heaters mounted on the back or sides of the tank above the substrate level, so that its heating the air inside the tank. UTHs aren't designed to heat through the amount of substrate hermit crabs need, and they can also become a fire hazard with the amount of substrate.
*"Hermit crab heaters" are tiny UTHs that are too small to make a temperature difference at all.
Humidity: 80%. Higher than that isn't detrimental to them, but it can promote mold growth in the tank. They breathe through modified gills, which means they cant be in dry air, but they also cant be submerged in water for long.
Food: Fresh and dried foods, and its best if you dry the foods yourself to make sure there's no bad preservatives. Some foods are toxic to them, so doublecheck on the internet to make sure. Hermit Crab Association has a safe food list and an unsafe food list. They need lots of animal protein in their diet, or they'll start eating each other for it. Mine love shrimp, lobster, fish, scrambled egg, cooked unseasoned chicken, cooked unseasoned hamburger the best for proteins. They also love fruits and veggies and things like dried maple leaves, millet and chia sprouts, etc. I make my own dried food mixes for them too, they're basically the same thing except for a few ingredients, but presented in different forms and that's enough to keep them interested because of variety. The 'chunky' mix is dried papaya, dried pineapple, dried cherries, dried banana chips, raisins, dried corn, dried peas, dried green beans, dried carrots, millet seeds, pecans, walnuts, dried river shrimp, dried crickets, and dried mealworms. The 'powder' mix is all of that, minus the fruit, and with added oats and cuttlefish bone. Mine also love organic honey as a treat! They always have a cuttlebone in there too, because calcium is important when they molt especially
*Avoid commercial "hermit crab" foods, they have harmful ingredients in them despite being marketed for hermit crabs.
Water: Two dishes, big enough for the biggest crabs to submerge in to exchange the water they keep in their shell to help them breathe, with a ramp to make sure the littler guys can get out once they get in. One fresh water dish, one salt water dish, both treated with dechlorinator. The salt water needs to be marine grade for saltwater fish because it has all the minerals, etc that oceanwater has. Instant Ocean is the salt mix that is most recommended.
*"Hermit crab water" doesn't contain all of the minerals of actual sea water, which land hermit crabs need to survive even though they're terrestrial, and isn't appropriate to use despite being marketed for hermit crabs.
Shells: This is very very dependent on what species you have, since different species like different shells. Caribbean crabs like turbos, Ecuadorian crabs like nerites, etc so I can help you better with this when you find out exactly what kindof crabs are available in your area. Caribbean/PPs are generally the most common in most places, though. There are a bunch of exotic species you pretty much have to order from somebody who imports them from suppliers in the native countries, but her business hasn't been getting good reviews lately :C But, each crab needs 3 - 5 appropriate shells to choose from at any given time.
*Painted/glow in the dark shells are toxic to them as the paint chips off and they eat it, and the paint makes the shell unbreathable through the microscopic pores in the shell that get covered and sealed with the paint. The process of getting a crab into a painted shell for the pet store is also extremely cruel. I wont go into detail if you don't want me to.
Decorations: They need hideys, and be careful of resin hides because some of them will give off a nasty chemical smell when exposed to heat/humidity like Percy's t-rex skull did and those fumes are very harmful to crabs. A lot of people use hollowed out halved coconuts with doors cut into them, but they can mold and might need to be replaced. They're also climbers and need things to climb~ Cholla wood is especially popular, and those reptile vines that are foam with wire inside and you can bend or twist them work great too. Hanging plants work too, but they like cholla and vines better. Mine also have coral and marine fossils that were disinfected and they LOVE them! They also love moss pits, which is basically a tupperware or shower caddy filled with moistened sphagnum moss. They eat it, they dig in it, etc You can also make second levels for them out of turtle docks, lizard ladders, hemp hammocks, locker shelves, etc so that they have more room to climb and walk around without being all up in each other's business.
*Painted plastic cholla wood isn't suitable because the paint doesn't hold up to the humidity and chips and peels off, and its harmful if the crabs eat it.
Companionship: They're very social and cant really be kept alone. They need at least one member of their same species with them, but a group of 4-5 is a good minimum as long as you're not overcrowding the tank. General rule of thumb is one crab per 5 gallons, but the big ones need 15 gallons per crab (and at least one foot of substrate).
I feel like I'm forgetting something, so if I think of anything else I'll be sure to let you know