Over the years I've always transferred my number when moving to another network.

I've gone from O2 to Three to TMobile to Vodafone.

If I wanted to leave Vodafone to (for example) go back to Three, would that be ok?

Also out of interest, who owns the number?

yes that would be fine. just do it in the usual way.

i believe it's still the case that ported numbers are still in some way routed to the original network and then rerouted, though they were looking in to changing that so maybe they have already.

I'm not sure who 'owns' the number, but I would suspect not you.

The number is the way to contact a subscriber and so I guess it comes under your current contract. Therefore, if you break the terms of your contract your current provider can close the account and disconnect the number. I doubt you'd have any come-back on that, though they wouldn't give away your number for quite some time (if ever). You'd have some trouble in getting the number back after a year, though!