Of course LHR is a "hub" airport, in fact it is a triple hub (for now) with BA, BD and VS based there with hub operations. Triple hubs are rare outside the USA, the only other that immediately comes to mind is Bombay (AI, IT and 9W).
The reason that LHR doesn't operate a system of flight banks or a ‘wave’ system like CDG and DXB, and not every hub does, is partly because of congestion, but also because of the the high number of frequencies between LHR and some key destinations on most days, and often on several airlines.
A few examples (these are maximum frequencies):
5 flights/day to BKK compared to 2 or 3 from AMS, CDG or FRA
6 flights/day to BOM compared to 1 or 2 from AMS, CDG or FRA
5 flights/day to CAI compared to 2 from AMS, CDG or FRA
10 flights/day to DXB compared to 3 or 4 from AMS, CDG or FRA
8 flights/day to HKG compared to 2 from AMS, CDG or FRA
27 flights/day to JFK/EWR compared to 5 to 10 from AMS, CDG or FRA
5 flights/day to JNB compared to 1 or 2 from AMS, CDG or FRA
3 flights/day to NBO compared to 0 or 2 from AMS, CDG or FRA
5 flights/day to NRT/HND compared to 2 or 3 from AMS, CDG or FRA
7 flights/day to SIN compared to 2 or 4 from AMS, CDG or FRA
4 flights/day to SYD compared to 0 or 1 from AMS, CDG or FRA
The same applies on other north American routes as well. There are also more third country carriers on many of these routes because the UK is more amenable to fifth freedom rights than some other governments.
That said, it explains (together with the paucity of available and affordable slots) the lack of overall destinations from LHR compared to the other 3 main European hubs.
Geographical location within the continent is irrelevant when it comes to trans Atlantic and far east traffic - the four main European hubs are relatively close to each other and American ones are all over the place.
However, cultural ties, trading and linguistic links and, of course, politics are relevant. Compare the spatial distribution of destinations in Africa for BA and AF, or in the West Indies between BA/VS, and KL. Look at the comprehensive route network of IB and TP in South America and BA's in north America. LH‘s destinations in central and east Europe tells another story.
As for LGW, it is more of a “focus” airport for BA and VS than a hub, as because their LGW routes are mostly origin/destination leisure traffic. The “no frills” carriers do not operate transfer systems - all their traffic is origin/destination, even if pax are physically changing planes. So although LGW is U2’s main base, it is not a hub airport for that reason.
Hope this helps you settle a score, Jabird, you are obviously correct. The only way LHR ceases to be a hub is in Silver’s dreams, and everyone else’s nightmares, where all the airlines have deserted LHR for a sandbank in the Thames!