MoD or UCL
I believe .uk was the third top level domain to be established after .edu and .us. This predated dns and would have been in 1982 or 3.
.uk was run with a hosts.txt file and the first sub-domains being either ucl or mod.
dns came in in 85 or 86 and the first sub-domains in that were copied from the UK NRS from the X.25 world (ac.uk, co.uk and mod.uk) so there probably wasn't a first dns sub-domain for uk.
This work was done by UCL CS and at least 2 people directly involved are still there.