Statistically speaking, more crew earn the BA rates of pay than any other, so would that not dictate how much UK employers pay crew at LHR and therefore the market rate?
In that case there will be no issue if BA drastically reduce their terms and conditions for cabin crew, since they will still be getting paid the market rate. In fact using this argument BA could pay minimum wage and justify it by saying it was still the market rate.
The fact is that at LHR there are two other airlines, one mostly shorthaul the only longhaul. Both pay about half what BA pays.