Really? Not on any of the flights I've seen, which have been completely full up front as well as at the back.
It was true out of LHR until early September. However, something has drastically changed since then. In the last 6 weeks I've had two "last seat" upgrades (on 777s nonetheless) and one missed upgrade on a 767 which never happens as a 1k on a B fare.
Something at LHR has really picked up in the last 8 weeks, and C cabins are quite full lately.