It's a pseudoscorpion which is a type of arachnid like spiders, mites, harvestmen, scorpions etc., each in an order of their own. All pseudoscorpions are small - at least European ones are. They're predatory, hunting small creatures like springtails. Occasionally they attach themselves to the legs of flies to allow them to disperse to new areas.
I used to have a key to UK species but seem to have lost it now, so can't help with a more precise ID.