Hi Dan,
I think the Club Europe seats are actually 3-3 on the 737, but BA block off the middle seat – I sat in one of the Club World convertible seats in the summer as the 737s have about 10 (?) rows of these then the rest normal to allow for differing business class loads.
Stansted’s a niceish airport, in comparison with Gatwick for example. However in the early morning rush it can become pretty crowded and security can sometimes be a bit of a queue, but I wouldn’t expect this to be so bad at the time you’re flying.