Strava **** up royally. We parked up and stayed at Heathrow T5 Holiday Inn the night before the ride, then rode to Slough, got the train to Paddington and rode to Trafalgar Square from there. I stopped Strava while we were on the train, and it was fine when we got to Paddington. However, by the time we got to Trafalgar Square it was on 55km. I've lazily clocked the ride on google maps since getting home, and I can provide the mapmyride maps which we loosely followed to Newhaven, and strictly followed on the other side of the channel.
The weather was shit in London, so I haven't even got a start photo. But here's a celebratory success photo...