I would really appreciate some guidance on the following question regarding an upcoming LHR T5 transfer. I have an ex-DUB trip to the US coming up, specifically DUB-LHR-DFW, all on BA metal connecting through T5 in J, with a ~2hr stopover in LHR. My question is as follows: do we need to clear UK immigration at any point? There is a possibly my American wife's new visa will not have arrived physically (although is already approved), and I had assumed when booking this wouldn't be a problem as just connecting through LHR shouldn't require entry into the UK. However, I'm now worried as I read in this helpful guide that "arrivals from DUB do not require passport control at LHR" - which suggests you would need to have the right to enter the UK before you got on the plane at DUB, even if you are simply connecting straight through.
Any clarification would be much appreciated. Thanks.