Upvote:2
From comments:
A married couple without kids is still a family.
Upvote:4
The gov.uk website says
You must ... move through passport control together if youβre in a family
https://www.gov.uk/uk-border-control/at-border-control
and
To avoid delays, when you arrive at UK passport control, please:
* stay together when travelling as a family
https://www.gov.uk/government/publications/coming-to-the-uk/faster-travel-through-the-uk-border
They don't define what they mean by 'a family'.
I suggest that you approach the desk together. If this is wrong then the worst that will happen is that the passport control officers will ask you to go through one at a time instead.