Hi!
I'm aware of 2 pw sceneries of EGLL: UK2000 and Aerosoft.
I would just download and try the demo version of Gary Summon's UK2000 Heathrow scenery V3. At least it's rather new and up to date and you can try if it is FPS friendly enough for your machine and your needs before buying it. At my FSX FPS are OK (not to say "good"). However I have ORBX England installed and a lot of AI traffic which is probably the biggest FPS limiter. Didn't buy it yet so far, just tried out the DEMO.
There is a now version of Aerosoft's "Mega Airport Heathrow" in the pipelines. But as you might have noticed it can take some time there to be released there. And they are not exactly well known for good FPS performance at big airports. I would certainly not grab their old outdated version.
The fact that EGLL wasn't included in the poll as well as the fact that there are 2 pw sceneries availible in a quite reasonable or good quality makes me think that FSDT has no plans for EGLL.
edit: Your specs are a lot better than mine (i5, not overclocked). So you shouldn't have too many problems with FPS at the mentioned airports.