You are measuring it as right as you can. The ISO measurement refers to the bead seat diameter, which is not the same as the external diameter. The hub centre and sprint rim also throws off the measurement a little too. While the tape measure doesn't lie, you will have to trust us that the old Mavic sprint rims are the same size as today's common 700c rims for tyres and smaller than 27 x 1 1/4.
https://sheldonbrown.com/rim-sizing.html
https://en.wikipedia.org/wiki/Tubular_tyre
It's not safe to have a too large wheel in the front of a bike. There needs to be a margin of sufficient clearance for buckles so you can still control the bike - this is a primary concern.
Hill Climb events is something very British. It was common practice to make the bike as light as possible and use fixed gear. When caliper brakes with Bowden steel cables first started to appear on bicycles they were usually on the rear. Eventually this was proved the wrong way, and for fixed gear riding having one front brake is the now default. I can't say exactly when this transition started to happen.
You mentioned that he would have been about 20 years old - at 20 years old you would have been tinkering with all sorts and being inspired by the good racers of the day. The CB probably had steel chrome rims originally which compared to the Mavics would have felt like carrying a boat anchor up hill
..... why would you want to keep them?
Joking aside, it's always difficult to nail down the history of these vintage bikes. The CB is a bit more involved, probably more costly and will require some patience to get going. The Carpenter seems more straight forward.