Are new UK iPhones unlocked

If the iPhone is purchased at full price as carrier-unlocked and without a carrier subsidy, yes, it’s carrier-unlocked.

As for transferring cellular plans, that can vary by plan and by carrier, as some charge for transfer and some don’t, and as some existing plans can cover 5G and some don’t; ask O2 directly.

The above is based on the assumption that you are in fact buying directly from Apple. The phones sold by other retailers in the UK are not unlocked. They will lock to the first carrier used.

I don't believe this has changed in the last year or so, but if it has, I'm sure someone will chime in and correct me.

I have heard, but cannot confirm, that John Lewis sells unlocked phones also. But don’t take my word for it. Carphone Warehouse definitely does not.