50mph will be fine, 15 would not. Keep to the nearside lane.
70mph is the MAXIMUM speed you are allowed to travel at, NOT the speed you have to travel at! At 50mph you will be up there with the caravan towers, trailer towers etc and in the nearside lane you will not be a nuisance as suggested by some one. On any road, the speed limit is just that, the limit, not what you have to drive at.