It's not a UK provider, I probably should have tried another forum but figured many UK voipuser people might also use them or similar for a US incoming number.
The porting in the US is much more ahead of the UK porting from what I've read. It is "mandatory" that they port it out if you request it, but... with VoIP I have still heard of problems since many providers don't "own" the numbers they assign. Voicestick shows a "port-in" option on their website though so if they allow ports in and not out I'm pretty sure that's a major no no.
Went ahead and attempted the port today and should, theoretically, know if it was successful in 24 hours. Will follow up just to finish the thread and leave the outcome documented for the benefit of others who may be in the same situation.