LN-KGL
One factor to bear in mind is that, at LHR, long-haul seat factors have generally been higher than those for short-haul. So in comparing LHR with other airports you do have to make an allowance for differences in traffic mix.
Of course you are right to point out that LCC seat factors will generally be higher than for legacy carriers, which will tend to offset this.
It just goes to show that comparing seat factors across airports can be hazardous, and that there is a danger in comparing apples with pears. Heathrow's hub status might be expected to result in a slight lowering of seat factors, particularly on short-haul, given the critical importance of frequency at a hub with a high proportion of business traffic. Conversely, an airport with a predominantly leisure traffic base (especially one with a lot of LCC traffic) might be expected to have high seat factors due to limited frequencies and lower average yields.