There is apparently a British equivalent to the US FOIA. If you're interested in pursuing this, then you're going to have to do some research on the UK laws.
However, in these cases, blind shots in the dark are not the best approach. Do some research ahead of time and try to figure out exactly what you are looking for and who is likely to have it. Sometimes you don't have to make a formal request at all and documents are already declassified but in a collection, or simply not made public. So if you do some legwork ahead of time you may dramatically increase your chances of getting the information.