I don't know what the smallest aircraft into LHR is now. I know there used to be regional services using turboprops; I know of at least four that have been discontinued, either because they were financially unviable or because the airlines concerned were offered a lot of money to give up their slots.

LHR is a hub - but most feeder traffic comes from other European countries rather than the UK. Those of us living near the smaller regional airports tend to transit via AMS or FRA rather than LHR... also, these days there are lots more direct flights from regional airports to European destinations, so for short-haul travel we don't need to transit via major hubs anywhere near as much as we used to.

You don't say what your nearest airport is, but if it is served by a *A carrier you may well find that catching your UA flight from somewhere like FRA may be your best bet.