Pros
In my experience, everyone's friendly and approachable. People are there to do their job well and take the time to help each other. There's no political scheming, machismo, or culture of being the first in and last out. The standard of the work is high and people are given the time to do things right. There's a decent range of benefits, they're open to flexible working hours, and the annual leave allowance is good. It's a small enough company that you can get to know almost everyone, and it's easy to get involved in one of the groups (R&D, the TAGs, or social committee) and make a name for yourself. Equally, it's big enough to have a lot of experienced people around.
Cons
It's not a place for anyone who wants to slack off or blag their way through the job - if you're not good then word gets around, and the project managers won't want to work with you. The geographical divide between the Heathrow and London offices means there's not enough crossover between the two sides of the company.