Hello
jeep1
That is what I always do when connecting at LHR. Should be no problem as long as:
a: You are entitled to land in UK with your nationality/passport/visa
b: You have ample connection time (remember t-35 for conformance at security - I always go back inside t-60 to nab a drink in the lounge first)
c: You have cigarettes on you - You don't want to be paying the over inflated airport cigarette prices
With all of the above, it also saves you from the scrum of flight connections and even if I wasn't going to have a cigarette during my connections, I would still choose to exit via the border & customs and re-enter from landside.
Happy & Safe travels
S