Ok you PM is off, anyway to go into weapons, there isn't a particular disciplline which is overly prefferred. Depends on what type of weapons also. I worked at MBDA, they purely design and manufacture missiles. In this case, aero, mech, electronic, chemical and stuctural engineering are all considered useful. Depends also on which part of the system you want to work in, explosives/payload, delivery systems, navigation etc etc. I actually was on the general engineering course at Cambridge when I did my placement with MBDA, and they seem to like general engineers as you have the ability to draw together different aspects of engineering for a particular product and these are the people who generally make it to the coveted project management positions.
The job is not as glamourous as it seems at first as the systems you are developing are extremely secretive so at the early stage of your career you won't be exposed to a lot of the exciting things. However, as you start rising up you will get more responsibility and your level of security clearance will rise and so you will be privy to a lot of top secret material, though sadly you need to keep it all to yourself.
As an engineer, it can be fun as you will be given the best tools, instruments and systems to play with and have a lot of job security with pay rising fast. There is a programme of constant training and so you keep learning new things and opportunity to travel over Europe as the major weapons systems these days are being developed in collaboration by the big EU states (UK, France, Italy, Spain and Germany mostly). There are excellent exit opportunities as I heard of people at my work place who went on to do an MBA and then were easily snapped up by investment banks and other financial institutions and getting paid very large sums of money. So you leave your career very open.
So my advise: go to the top unis for engineering, these are Cambridge, Imperial, Southampton, Nottingham, Loughborough, Bristol, Bath, Brunel and a few others. Do a course you like, and if you're finding it hard to decide, mechanical engineering is a no brainer because that will get you into any engineering job. Whilst at uni, you can apply for internship positions at the defence companies, a lot of them have structured internship/graduate programmes. MBDA, DSTL, BAe, Thales, Raytheon..the list is big. Its not particularly too difficult to break into, as its glamour would suggest, get a 2.1 or above and a good reference and some good practical experiences and you should be fine.