As a UK resident UA flyer, I think the UA programme is about the best for the OP's needs:
1. You get 100%PQM for miles flown in Y, even the cheapest Y
2. Gold gets you E+ which, whilst absolutely not PE, is an awful lot better than regular economy as every inch matters.
3. When you eventually earn enough miles, redemption for long haul is superb - only taxes to pay, availability is very good and many of the partner airlines are first rate. Whereas BA's programme has value in short haul, UA's is definitely long haul.
4. When you stop flying UA, it should be fairly easy to keep UA miles alive as, even if you don't fly any *A carriers at all in 18 months or rent from Hertz or stay at a Hilton, trivial things like donating some miles to charity will extend the life of all your miles.
It's not nearly as good as it was but, for a traveller to one of the few destinations in the USA which UA serves from LHR, it's much better than BA. Just don't expect upgrades to J if you are only Gold.