Missed my other warmup race (Marshman) with tennis elbow, but that's eased now - looking forward to this one. Apart from the early start! We're staying in Swindon Saturday night if anyone fancies meeting up for a lemonade.
Suggestion from my OH, who was a nurse for 20 years - get an opinion off your GP. If he really thinks its dangerous for you, then you'll know - if not, you have some ammunition. Still hope too see you there!
If slightly smaller, I'd also look carefully at the reach - make sure you aren't stretching to reach the handlebars. My wife rode the wrong bike (a men's road bike we got off EBay) for a year or so, really didn't get on with it, but put it down to her inexperience with road bikes. It was only when we sat her in a bike shop on a women's-specific frame (Giant Avail) that she realised how badly it had fitted her. As FF said, essential to try before you buy, preferably with a friend who knows something about bikes.
Good luck at London - I did it last year, it was fun!