Training this winter

You could argue that a shortish central London commute is useless for either in any case, so much stop and start and no bits open enough to go hard or do intervals, you usually have to ride at a speed which is safe to flow with the traffic (it's not always gridlocked, and you are usually mixing it in bus lanes) so even going steady can be dangerous at times. Staying alive often comes before training unfortunately.
I haven't found much riding in town to be any use at all, except at night where it's clear enough for a run at things, compared to 45mins on the rollers it's awful. It must be so much worse for the 9 to 5'ers.

Some valid points there, I think you just have to accept what time you have and do the best you can with it. A girl I have been coaching this year commutes to and from work in central London and was doing the same last year. We got her to improve massively this year by cutting her down to every other day and tubing in on the rest days. She was fresher and therefore could work harder on the days she did ride in.