Keeping UK mobile number in Spain

I reckon the more typical solution for a Spanish SIM + UK SIM is two single-SIM phones, or one dual-SIM phone. I've found both options to be pretty convenient, although dual-SIM is a bit more elegant. It's quite a restriction to insist on one SIM only, and I'm not sure why it's necessary.

I think all my UK banking apps accepted my Bulgarian cell number, so I don't even need a UK number to access them.

I also have Skype numbers in USA/UK, and Skype works for both calls and SMS messages. So this is also a solution for banking apps if they won't accept my BG cell. My USA banking app won't accept a non-USA number, so my Skype number works for their security message. But I think this is the only one that has this restriction.

I would think Xpatfone is probably working quite Skype-like with VOIP via your local Spanish SIM data allowance (or WiFi). But with the advantage that you can port your current UK number over to them. If keeping your number is a big deal, then maybe it's an interesting option.