Chemical warfare research, microbiological warfare, methods of protection from both, biochemical research specifically related to warfare and terrorist attacks, armour development (we are incidentally at the forefront of methods of protecting tanks from weaponry), immunisation research, explosives research, robotics, battlefield communication methods, security. That should give you a general idea.
If you have a few spare moments, look up the official websites of Porton Down and Fort Halstead etc via the main Dstl website. You may find the main QinetiQ website enlightening too. Rest assured, UK scientists at these establishments and others are regarded the world over by their colleagues as being at the pinnacle of their profession and expertise. It's not uncommon for USA military scientists to consult scientists over here for over definitive answers to their problems.
Incidentally, to be fair to them, USA military scientists have unparalleled expertise in the field of the so-called "Star Wars" technology.