Can you transfer holdings from an ISA account into a regular broker account in-kind?

Two questions here, the most important one is actually whether you can find another ISA provider that will take your business so you can protect the wrapper status for the long term. I'd ring round/explore the big UK brokerage only companies who don't have quite the size of the regulatory headache Barclays may have, and/or may have setup ways to work on this as they're much more nimble and this is far larger percent of their core business.

On the main question, and as you've surmised, very little info on this as it's not something people would ever normally do. Virtually all brokers offer share transfers between ISAs (usually with a fee though, as transferring ownership of shares is a bigger headache for them than sending cash), so even in normal times you're usually better off selling positions before transfers between stock ISAs depending on exact broker and the size of these fee(s).

On moving to a non ISA account, I would guess they might be fine to do this for other brokers who offer ISAs and regular accounts, as it will be all setup in their systems already, may be some issues transferring to non ISA offering broker as won't be on their systems, but one you can only really answer talking to the broker(s) in question (or someone with a deep knowledge of the ISA plumbing jumping in here).

If you can't find anyone who will let you open an ISA under your current status, by far the simplest thing is just selling the whole position, cashing out the money and depositing into a new broker.