Pros
Some great technology, if you can get to it. Nice offices. Friendly, hard working, clever people remain in the UK. A highly progressive company who will help staff with specific needs and promotes diversity.
Cons
Work life balance is a joke. They never push back on customers. Career progression is impossible, unless you're a good manager. Even then they may reorganise your position away. Wages are stagnant. Competition from far east companies and among the remaining western companies means virtually everything is offshored. There is a danger of skills rot due to enterprise customers often being conservative in accepting new technologies, and the long shelf-life of certain products. Change: this doesn't contradit the previous comment, but there is always a lot of change. New solutions, but often based on "old" technology.