As someone who does non protected connections at LHR a lot..I expect you have about a 25% chance. Especially when you realize that if you are not at the gate by the LH deadline (which is NOT "take off time") they have every right to say "Oh you didn't show so we gave your seat away)

Is it possible? Anything is possible. You have to assess your own risk tolerance, your physical ability, your familiarity of the ins and outs of a particular airport.

The post above me is on point. OP does NOT have 70 minutes but 70 minutes less gate closure time - is it 15 minutes prior to scheduled takeoff time?

Also, there are currently constructions at a number of gates at T2A, so there is a nonzero chance that intra Europe flights will depart from T2B; in fact, I just flew one from T2B from LHR to OSL earlier this week around the same time frame (7am), and there were multiple aircraft also leaving from T2B around the same time. So, another factor to add would be the possiblity of a long walk from A concourse to B concourse, in addition to arrival time of the VS flight, seating position, walk speed, knowledge of LHR, is fast track security available to OP, etc.

I am guessing the specific LH flight OP is choosing might be cost related. Later flights might be more expensive. If costs are similar, I would suggest choosing a later flight to minimize issues.

Personally, I would do it, but I am also a high risk taker having done many "connections" on separate tickets. My most recent "accomplishment" was less than 16 minute "connection" from D11 to F22 at SFO, and I made it in 12.