77minutes should be enough time for you to clear Immigration as long as the flight is not delayed (or too early!). If the LGW flight gets in late it bumps up against quite a few flights from the Caribbean and MUC and the immigration lines can get quite long (if its early, you bump against the FRA flight). Best bet is to try to get seats as forward as possible in the cabin, and walk briskly to get in line quickly. Also make sure that all of your paperwork is filled out correctly, because any inaccuracies or incomplete fields will guarantee you'll miss your connection. Baggage handling has gotten much faster at CLT, but probably not to get all the way to UA in Concourse A, but things could be firing on all cylinders and all will be fine.