Finally got through on the live chat and was able to cancel before the 14 day cooling off period ended. They couldn't confirm anything in writing, wouldn't send me an email.
48 hours later still nothing from O2 but logged in to see services had been removed. No idea what, if anything I'm going be charged for and as I now no longer have services, can't check. Am having to block O2 via my bank but as I had filled in a Direct Debit form, might not be able to.
What a terrible, terrible customer experience from start to finish.
Your MyO2 access ends pretty sharpish when you leave O2.
I would advise against removing the Direct Debit at your bank yet, as you will have a worse job trying to pay O2 if there was a charge after return that they could not collect (enough to get a flag against your credit record!).
Keep an eye on your bank account and raise it with your bank if O2 take money you think they should not have, until everything is settled. My 0.2c worth.