My enthusiasm (if that's the right word
) for Heathrow's WebTrak is slightly dented by the fact that the module that displays individual aircraft tracks has been down for most of the last week.
That said, even when it's up and running, you will look in vain for tracks of the spook missions. I believe Heathrow got their knuckles rapped for publishing them in the past, and they have suppressed them from the public system for the last few years.
Nor do the flights appear to use Mode S at all, presumably relying on Mode A/C and primary radar to allow ATC to keep them clear of LHR inbounds.