I am no flight tracking pro, but FR24 suggests the holding over Scotland delayed the flight by approx 25 mins.
Subject to the accuracy of FR24 data, the flight arrived on stand 301 at LHR at approx 14:00 hrs UTC
Stand 301 previously seems to have been occupied by BA2610 to Naples, which didn’t push back until approx 13:45 hrs ……… so all other things being equal, if JA43 had not been delayed, it would have arrived 15 mins or so before stand 301 was available.
So are stand allocations at LHR (or maybe just T3) constrained such that another stand would not have been available for an earlier JA43 arrival, so the JAL flight was asked to hold in relatively ‘empty’ airspace, rather than have it arrive earlier and have it clogging up taxiways waiting for stand 301 to become available ?