Three Challenge

Apparently 3/Three don't accept that I own the phone. Worrying; I suspect this is true for all mobile phone companies. Because I don't have the purchase receipt or the details of where it was originally bought, they won't accept that I am the owner. So what?
Well if the original owner phoned them up, read out the IMEI and said the phone had been stolen, they would block the phone. Fortunately they didn't register it, but I can't register it either.
My other motivation is that 3 will unlock the phone for £15.53 (apparently) whereas all the other places want about £24. I shan't be using 3 once the phone is unlocked ...
The system beggars belief to my mind; they sent me a SIM to my postal address, they can see from their network that I am calling from my handset (with the IMEI), you'd think they'd want to encourage people to register. At worst they've got some personal details they can bundle up and sell on (email, phone number address etc.) at best they've got that plus marketing data about the second hand market for their handsets, something they probably need to know about if they want to sell more new handsets.
Hopefully the seller will be forth coming with the store and the date of purchase which can then be used to confirm to three that I actually own the handset now and so it can be registered to me.
(and then promptly unlocked and switch to Giff Gaff :D).