In all honesty, they don't intent on making outbound calls, it's just to retain the number (and accept incoming calls). Do you still require a calling plan to receive inbound calls, too?
No. I do have a sipgate basic number I set up years ago for testing with no credit which still accepts incoming calls, I went with AAISP as at the time sipgate had discontinued number ports on their free service - these were later reintroduced.
AFAIK with sipgate you have to add the minimum amount of credit (£10) so they have validated credit card details they can charge the balance of the number port fee to it.
Interesting, so a monthly charge to keep the number active - sounds more like the kind of thing thy want to do (and just receive inbound calls, rather than make).
Not with sipgate, they did have a phase ~5 years ago of killing off inactive numbers, having an ATA or SIP phone actively connected appears to be enough to be classed as active.
There are charges for number blocks / transit charges between providers for ported numbers so the VoIP provider needs some mechanism to pay for that - some may charge a fixed recurring fee, others may take a sliver of the per minute call charges to cover it (so people who make outgoing calls will pay slightly more, effectively subsidising people who only receive incoming calls).
I think the new line with broadband first would be ideal, but I'm assuming that would require Openreach to come out and put in a new line (pull a wire from poll etc.?)
An engineer would have to fit an additional master socket, if the current installation is old enough there may be a spare pair in the existing cable otherwise a new drop cable (for overhead fed properties) would be fitted. If it does require a new drop cable you could potentially have the new master socket installed in a more useful location than the current one, there are restrictions on working in premises with the current virus situation.
How long do these ports usually take? I know you have to fill out a form of some sort
The figure quoted is typically around 7 working days, it is often less but could be more given the current situation.