Several options off the top of my head:

1) Get a temporary international plan add-on (or whatever the specific term is for you carrier.) Not usually that expensive.
2) See what the "pay as you go" rates are for international calls. The most expensive are usually around $2 / min. YMMV.
3) Investigate getting a UK SIM. There used to be a company who would have your SIM ready for you to pick up at LHR (or other major point of entry) and you get the number when you "book" it. With Covid etc, not sure if they're still around.
4) Probably others I haven't thought of yet.

Best option is to compare #1 and #2. assuming it will be a single relatively quick call.

rb211.