Hi all,
As far as I'm aware there are no poisonous crabs in UK waters, and the most common species that we have make great eating. As Den has already mentioned, Spider crabs which fishermen can't give away in this country are sold for goodly sums in Spain and Portugal where they know a thing or two about seafood.
As far as preparation goes, crabs, as with all shellfish needs to be cooked as freshly as possible, however they will keep in the bottom of the fridge overnight (still alive) crabs should not under any circumstances be stored in fresh water.
When it comes to cooking, I land them straight into a big pan of boiling water (sea water if it's clean anough in your area) and give them about 15 to 20 minutes for big brown crabs and a little less for the spiders.
Pull the crabs out of the water and turn them onto their backs, You should be able to get your fingers into the carapace just below the mouth and pull the shell off the belly, this will expose the 'dead mans fingers' which are grey/brown coloured fibrous finger shaped things , these can be easily removed in more or less one piece, scoop out the cavity and you will see the brown meat, the white will be in the claws, which you can crack open and pick out with a crotchet needle or similar ( I'm never without mine!!).
The meat can be eaten as it is, with hot butter drizzled over it, or it can be mixed with mayonaise or a curry sauce, whatever tickles your fancy.
For those of you who are travelling up to Islay, the seafood available is fantastic, if you get a chance to go for a pint in Bowmore, and if you can collar one of the local fishermen they're usually more than happy to give you a couple of huge brown crabs for the price of a pint or two (No jokes please about getting crabs for the price of a pint!! I'm offended terribly easily!!

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Hope that this helps.