Well in my company (and Antiguas) the guidelines for declaring a fuel emergency are clear:
PAN if you think you might land with less than reserve.
MAYDAY if you know you will land with less than reserve.
In the case of committing to LHR one is entirely within the letter of the law and is not endangering the aircraft provided the EAT/No Delay (yes we know it means up to 20mins) is reasonably accurate. Provided it is, then the situation is not an emergency. However if the aircraft ahead is a bit slow to vacate then a nervous ATCO sends us around, our calculations are done for and we are immediately into a Mayday scenario.
Antigua wasn't asking to jump the queue, but merely emphasising that he'd like more seperation in order to prevent you having a very disrupted morning.