I'm registered with a good number of employment agencies, but so far I've only ever been asked to provide proof of my identification from one of them. This agency has not contacted me to discuss anything with me, but have asked for lots of forms to be completed. What I would like to know is. Is this is a legal requirement, and if so, how do you guarantee that these kinds of details are being used/stored correctly? I know it would be very easy for a company to say that they are, but it still niggles me nonetheless.

I'm fairly certain Tomkins is right, it's to do with proving that you have the legal right to work in the UK. From previous experience they usually check your identification once you've gained the position or at interview stage. I suppose that the employment agency could be trying to streamline the process and getting as much stuff out of the way upfront. If you're worried about your data you could always ask if they have a data protection policy and ask to see a copy, but as implied in your post I don't think there is a way of actually knowing if this is being followed.

I concur with the others. It is a legal requirement that employers check a prospective employee's right to work. There may be other factors that would lead to a more intrusive check of your background - security clearance for certain jobs, police, care workers, or if the role involves management of operational security, anti terrorism controls and access to restricted information.