120.4
If an inbound aircraft tells you they have not enough fuel for a GA, if true about the fuel amount, then the situation itself must be a violation of British Aviation law / rules / regulations / JAR bibles.
I am sure you had no reason to doubt the fuel information given to you was untrue. You were then looking at a clear cut case of a violation which, owing to your capacity and perhaps good luck of having favourable circumstances, turned out OK. As an ATCO, were you not responsible to report the incident to UK CAA Flight Safety department (or wherever you report it in CAA)? At least the way it works here, looking it the other way round, if an ATCO does not report an incident, he/she has made a grave error. From your post it was evident that you only talked to the commander.
The CAA then would have launched full investigations as to the amount of fuel left & made a judgement was it a violation or not & taken corrective & preventive action. Or are these violations so commonplace at LHR that you'd spend most of your shift faxing incident reports?
Nevertheless I am sure you did a good job, don't get me wrong. Glad you started the thread, it's been excellent.
-headwind