I don't think this suggests that BA does not have a dominant position at LHR. 55% is a very significant position of dominance (in fact well above the level of dominance that most regulatory authorities would accept on almost any market) and in fact this suggests that BA has an even stronger position of dominance at LHR than AF at CDG or IB at MAD. The comparison with US hubs or even LH at FRA is difficult because those are not as attractive as O/D destinations as, say, London or Paris. In other words, ATL would be a much smaller airport had DL not chosen it as a hub and the same as FRA without LH or even AMS without KL. By contrast, LHR or CDG would suffer less because they serve the two most active metropoles in Europe.

* Note: UA at EWR is a bit different, NYC has massive activity as well, but UA is the only airport to focus primarily on EWR whilst most US airlines focus on JFK and/or LGW and most international airlines on JFK.