I think this answers the question I raised earlier today on here. Thanks to JinJin for doing some digging as well.
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Pretty sure you have to ring them and request it (or possibly email, in my experience they’re pretty good on responding).
I’ve had reasonable experiences with envirofone although they did rip me a bit with a 6 Plus I sold to them.
It's just a matter of timing. If all goes to plan and I can get a new iPhone XS Plus for delivery/pick up on 21.09 then I don't mind posting my current X a couple of days before. A 7 day extension would make things easier. I did try and buy a iPhone 6 last night on eBay for £150 but didn't win. Hard to know what to do...