Thames Valley Park is a sales & marketing office. There's a consulting services presence there too, but that really just supports sales & marketing. They do some cool stuff sometimes in the Microsoft Technology Centre though.
They used to do product development but nowadays it's all sales and marketing. The game testers the other commenter is talking about is centralized testing for Microsoft's remaining game studios. It's not very 'posh' either, at least when I was there, quite dated 90s looking offices. I think Microsoft development in general seems to heavily focused in basically the US and India. Exceptions of course, there's acquisitions like Skype and plenty of other examples dotted around but this place is hardly a 'campus' compared to Redmond or IDC.
They always seemed to have quite a lot of games testers there as well(used to recruit for contract MS workers for Reading and London)