Well I guess we have to agree to disagree....having said that, I agree entirely with the AAIB in the sense that a "PAN" call would have been more appropriate.
Personally would be happier committing to LGW, rather than heading off to LHR, for the reasons I previously highlighted, and I am of the opinion that it is the safer and more prudent option.
My own plan in an instance similar to this (after discussing this very issue with a few guys at west drayton) is to make ATC aware that at time XXXX a fuel PAN will be declared...not strictly Ops manual, but IMHO gives a heads up to the relevant ATC.