There is still one Tu154 a week into Heathrow T2, Rossiya (ex-Pulkovo) Airlines from St Petersburg on Saturday mornings. They changed over to 737s last year, and now have some A319s as well, but the Tu154s have come back this summer. It's usually in alongside the Aeroflot Il-96 used from Moscow on Saturday mornings.
There were once-weekly Tu154 flights from Heathrow to Leningrad (as St Petersburg was then called) in 1980. Has any other route out of Heathrow retained the same frequency and type over 27 years ?