Didn't you just ask this a couple of days ago?

Try a LHR-MAN return in H class...it's 20 more points than you need, but it'd cost you Ticket Price + £15 offline fee to only do H class one way and I doubt that's cheaper...still, you could call BA and check. I think a H class return is around £180. Or for a better day out, I can recommend LGW-BLQ for ~£190 in I class. Again, more points than you need...but more enjoyable getting them too.

Phil