My "red herring" earlier about comitting to LHR is something which requires a little thought, though I don't deny that to do so conforms to JAR ops.
Many people use their diversion fuel into LHR as effectively extra contingency fuel, routinely comitting themselves to a landing there. Almost 100% of the time this is fine, and they land safely. The problem arises when LHR is suddenly totally closed whilst they are at a late stage of the approach. I know that this happens very infrequently, but if diversion fuel is routinely used as contingency fuel, then over the years sooner or later someone will have a problem.
It is not true to say that this is the same scenario as being comitted to your diversion following a go around, since the chances of both getting the diversion and then finding that airfield closed are very remote.