Heathrow and Manchester some years ago. I'm not sure what the current situation is. They're definitely not used on arrivals, but I haven't been flying out from London lately.
And don't forget the return journey. I get backscatter scanned (at the gate) around 25% of the time when I travel from Schiphol, NL to Cardiff, UK (a journey I do often).
I’ve not flown to the US since 2003, but I’ve flown many times to other European and Asian destinations from Heathrow and Gatwick and I’ve never been through a backscatter machine.