Having written that some of the ones that I think should not have been started in the first place were the Brabazon, Princess, the rocket powered fighters and the Avro 730.
IMHO the money spent on the Brabazon aught to have been spent on an aircraft like the Britannia. The head start of 2-3 years would be useful.
The effort Saunders Roe put into the Princess, SR.53 and SR.177 and the associated effort by Armstrong-Siddeley on the rocket engines aught to have been put into helicopters and ballistic missiles IMHO. I'm probably being over optimistic, but it would have been nice to have a Saro helicopter like the OTL Scout/Wasp class in service in the second half of the 1950s. I'm probably being over optimistic on this one too, but I also think we could have had Black Knight and Black Arrow a several years earlier than OTL.
The resources put into Avros 720 and 730 would have been better used making an earlier start on Blue Steel Mk I IMHO. If the Mk I was further advanced at the end of 1959 perhaps the Mk II version might not have been cancelled and further development paid for with the money the UK spent on Skybolt IOTL.