Those placements are hard to find, but equally they're vital to your long term career prospects and nobody's going to help but yourself.
Somewhere like Marshall would be fantastic, but you're clearly aware that there are probably many more applicants to them than possible placements. The same will apply to the other big firms, because they're who everybody applies to - this doesn't mean that you might not be successful, since somebody will be, but the odds are firmly against you.
Where BAE is concerned, don't assume by the way that they are one big company - that's a fiction put about by their
PR department. Apply separately to the HR department on each site - and always follow up by phone. Do the same with Qinetiq/MoD sites (who historically have been the best at offering such placements).
However, my best advice would be to see what you can get out of the smaller firms - they might often welcome underpaid but bright slave labour for a year (that is what you'll be, but don't worry about it - it's still going to be very rewarding). I'd pull
CAP 475 (the CAA directory of approved companies) of their website and start going through it - look for the A1, E1 and E2 companies - those are most likely to be doing the sort of variety of work that will make your year rewarding and interesting.
Best of luck,
G