Can't see (excuse the pun) any reason why the Captain wouldn't return to LHR. VS are based there, might have had a spare crew unlikely but if not then the next day. Yes div to Shannon but unfamiliar airport increased work for one man if situ deteriorated.
Fact is crews come up with a host of reasons not to turn up for work. Once airborne however, no one would divert back to base without good reason. Hope the FO is OK.