A comment on flyertalk addresses this exact issue:
At LHR, it’s been ok about 140 times in the last 24 months for my
family doing just as you wish to do. Not one negative comment from any
UKBA employee about doing so.Even non-EU/EFTA/EEC nationals who have infants or toddlers are often
directed — late at night at LHR — to use the shorter/emptied out EU
national line at LHR after that line is cleared but the
non-EU/EFTA/EEC line is substantively backed up. See this at LHR T5
and T3.
Credit:stackoverflow.com‘
4 Mar, 2024
5 Mar, 2024