If the old boiler has a pilot light then it will be an atmospheric burner, it will only require enough gas pressure to make Burner pressure, for example 10mb, your new boiler will be a zero governer, these ideally require 20mb inlet working, but depending on the boiler can be as low as 14mb, more usual is 16.5 to 18mb. If your meter regulator is supplying 19-21mb working pressure and you then don't have enough working pressure at the boiler, then your Pipework is undersized. The engineer who carried out the survey should have carried out a pipe sizing exercise to determine if the existing pipe is suitable, he has said it is not and requires upgrading for a new boiler, if he hadn't done this and just fitted a new boiler, he would be liable to correct the installation error at his own cost. Sounds like he is a professional doing a good job, I would suggest you follow his advice