It is entirely possible to take a non EU passport holder who is a family member though the UK/EU channel. You have to use the manned desks which are there for people who can't use the e-gates (children under 12 etc), so join that queue not the e-gate queue. The manual desk used for failed checks at e-gates is not capable of handling non EU passports.
My wife is Ukrainian and still using a Ukrainian passport and we do this every time without issue. I have done this at LHR T5, T3, T2 and LGW many times this year so far. In fact it was a border force officer at the desk in T5 who told us we were eligible to do this after queuing for hours at the fast track non EU a year or so ago.

Obviously this only works when travelling with a british passport holder and I think you need a family relationship with the non EU passport holders.

Edit: I think it is fair to say that the EU/UK non e-gate queue at T5 is generally much quicker than the fast track non EU queue. Although once landing late at LGW with family heavy flights from EU then EU/UK queue was quite big and the non EU queue empty so I joined my wife in the non EU queue and went through immediately. I've never seen queues at T5 which would suggest the same would be true there however, the EU/UK queue moves much faster than the non EU queues.