I second the notion of getting a flip-flop hub, though I will add that I've ridden mine fixed all along and don't intend to switch to the ss. YMMV, however, and this gives you more options.

It certainly is simpler, what without derailleurs and shifters -- things that don't always do well in the cold. In other words, less chance for ball ache.

I doubt there's much in the way of hills in London. Hills would be one reason either to stick with gears or go ss.

As for the 2 bikes you mentioned (Wilier Pista and the new Felt Footprint), I'd think you could get something for a lot less than either of those, especially if you are willing to settle for used. FG/SS isn't for everyone, so you might want to check it out inexpensively first.

I sure like my FG, and I ride a comparable distance to what you mention.