Is this a reference to Speedbird 38 ? Last "disaster" at LHR before that was BOAC's 707 in April 1968 ... a tad previous to be GPS related.
It was suggested at one time that very high powered RF may have responsible for shutting down part of the fuel or FADEC system on BA038.... esp. as Gordon Brown (then UK PM) was due to fly from Heathrow about that time and there were concerns about RF triggered devices, so allegedly jamming was used.
Like most conspiracy theories it came to naught.
Can't see what GPS would have to do with it.