As already mentioned, if you regard the other person as your partner then you should use the UK queue anyway, and if there are jointly parented children under 18 years old involved then you all really must use the UK queue.

But leaving that to one side, the only area where T5 passport queues sometimes has issues is the NEEA queue (Not European Economic Area). I can't really see a pattern to it, so I suspect it's more about local staffing, school holidays, flu/covid outbreaks. Other terminals have much worse figures than T5, in their cases due to the way that flights from certain geographies all seem to land at once. In the last week - so a quiet time for travelling - only on Friday did T5 register as overloaded, with 50 minute delays at 18:45 hrs and then 70 minutes at 19:20 hrs . By 20:30 that queue was below 45 minutes, which is the service level for NEEA. That was the only incident in over a week as far as I can tell, there was a peak expected the previous Friday evening but it didn't materialise. Now I don't think waiting 45 minutes is acceptable, it should be below 20 minutes in my view, but nevertheless depending on the date of travel I would not see a risk of 4 hours as something you need to plan around, it's just so unlikely at T5.