Whats the best London bike shops?

Aww, don't knock Bromptons they aren't that bad. If you commute say from outer London, get the overground then ride either end they're great, or have a small flat etc.

get a real bike ride it and HTFU

Tests have shown that up to 16 mp/h, the small wheel is more efficient than a big wheel. Between 16 and 33 mp/h there is little difference. Over 33 mp/h the gyroscopic effect of the big wheel makes it more effective. Most folks do not go over 33 mp/h.

Source: 1984 Olympic Men's Road Race Gold Medal winner, Alexi Grewal during a conversation with Jeff Linder.
[Alexi owns a Pocket Rocket (a higher end folder)]

Note: 33 mp/h is more than 53 Kilometers per Hour! Even 7 time Tour De France winner Lance

Armstrong (arguably the world's fastest and best cyclist at the time of his last Tour Victory) averaged just shy of
42 Kilometers per Hour in his last tour despite (because?) of large wheels  and skinny tire
The world cycling hour record is about 56 Kilometers per Hour and there are 
probably about a dozen guys in the world that can even get close to this.

Remember that large wheels originated from a time where there were no such
thing as gears - witness the direct drive Penny Farthing bike. Now, the size of the
wheel can be optimized - and that optimal size is a lot smaller than 26" or more.

1. we dont have gears on our fixed or single speeds
   
2. heresy you will burn for this
   
3. all of that information is debatable, and there is a reason why people who make a lot of money to go fast ride tall skinny tires on the road.