FWIW, I've got it starting 133km in, north of Bedford as Graham has it. The AAA tool uses OS data in the UK which is of generally better quality than the STRM global data RwGPS is most likely using. Also the sampling method is chosen to match existing AAA assessments as closely as possible, so I wouldn't obsess too much over the exact climbing amount, which like measuring the length of the coastline is always going to be an approximation.
In other news, I've booked the Barry Premier Inn for Friday night, looks like the place to be, and will wing it with a bivvy bag for the others. Need to round TINAT to decide on the gearing but most likely keep the 73".