Without paying off device plan.
I’ve just got mine done.

Cheers mate, been waiting ages and kept getting told no

That’s interesting. I don’t have any issues with O2, other than their rather tardy implementation of 4G locally, but given the option to be able to unlock the phone while my contract is still running I’d like to take advantage of it, on a just-in-case basis; finding myself somewhere for an extended period of time without a network connection, and being able to get a cheap PAYG to use temporarily.
Any links to further info?

I did it through online chat

You just write your details in here –

https://www.o2.co.uk/apps/help/help?qid=1&q1=2&route=unlocking&case=Handset%20Unlocking%20Form

I’ve just spoke to online chat and it will be done in 72 hours. Way faster than the form.

Mine took 3 hours.