Well google is a good place to start, plus
http://www.companieshouse.gov.uk/index.shtml should have information on all companies reistered in the UK, i think you can get the financial info as well.
if you want tactics to use against them this should really be in underground but think keyloggers, PHP file browser trojans, log analasis, etc if your going to the dark side. Plus there is a free legal advice provided by a number of uk services like
http://www.communitylegaladvice.org.uk/ and you can just contact them and ask where to check the companies legal history and if you think they are doing anything dodgy you could ask about legal standing as well.
Just dont do anything electronic that can cause financial dammage to the company IF you want to be legally safe but snooping is FAIRLY ok.
Plus if you need a work history let me know and I can help you create something.
