After having my bottom bracket removed I've noticed the thread is full of rust and perhaps a bit worn. When I get the frame powder coated will their preparation process of baking it and shot blasting it sort this out?

Also the BB is a Shimano UN72 (68 x 122.5) and needs replacing. These don't appear to be manufactured anymore and I'm having trouble sourcing a secondhand one. So what are my options? Is it just the UN54?

Powder coating will only fill in the threads with the powdercoat, depending on how good they were at masking it off

Leave it for now, after it has been powder coated take it to a good bike independant shop like Bike Fix, Brixton Cycles etc, and get them to ream & face the head tube, tap and face the Bottom Bracket shell, and tap the bottle cage mounts, would budget £40-50 for that. expect that for any powdercoat job, regardless how good they prep the frame.

When you get the frame powder coated make sure thay cover the brake mounts, but be prepard to use wire wool / scotch brite on them after, and to tape , cover all other holes, sure they will do this, as the one you have chosen advertiese as having done lots of bikes before

For the BB, you can get UN54's still, these do the same job https://www.chainreactioncycles.com/M...?ModelID=17473, don't bother with 2nd hand, as it will have been used, and probably be no better than your failed one.

Thanks Jim, much appreciated.

A wire brush on a Dremel tool or other die grinder will clean the rust from the threads.

Before you drop coin on the powdercoat, have it chased to make sure the threads are still good. Otherwise you're just wasting your money.