That sign combination in Bottisham is rather notorious because both signs are wrong!
Even following the bizarre Sustrans mud bath route, it's 11.5 miles, not 15 (did someone not type a 1 at some point, then someone later think "it can't be 1.5 - they must mean 15" rather than actually check?) - and you can cut 1.3 off by using the direct B road (which is so-so and would benefit from a cycle track, for which there's enough space in the highway corridor).
Meanwhile, it's actually 7.5 miles by car, not 6 - but yes, that route would benefit from a cycle track too, but because there aren't many places on it, I reckon Burwell-Exning should come first.