LHR-HEL-LHR £342 (including a cheap hotel within walking distance) - 160 TP
HEL-LHR-OTP-LHR-HEL £607 (including a cheap hotel - again in walking distance) - 320 TP - bought via AMEX using £200 off £600 code that I had picked up on my Plat card - so £407.
1/2 day Friday to leave London, and arrive back Monday to work the afternoon.
I recognise these aren't a great price - but its convenient for me.