I'm assuming that since according to ITN the crew were advised of the separation of the cowls from the engines by ATC (seen by the CT) immediately after rotation, the natural option would be to return to LHR immediately, which presumably would have been agreed with ATC if no other problems were apparent that would otherwise have necessitated diversion to LTN or STN
Of course you'd go to LHR - no point in going to LTN if your cowlings are at LHR
To all considering the time aspect, very rarely, even Red LAND ASAP, is an all out attempt made to get on the ground as soon as physically possible. The only possible exception is a confirmed, unextinqushed, engine Fire. In the event you do want to get on the ground ASAP, 99% of the time it will be the briefed return to the departure airfield. Only exception I can think of is AWOPS when you have a pre-briefed TO Altn.
We will have to await what happened here, and when. But
if (and it is a big if) all cowls came off on TO, and the aircraft was flying fine, there would seem no urgency given a static situation and the rather too frequent times this has happened before. The interesting aspect will be what/why then happened to #2?