I think you're quite likely to run into pronunciation issues with both Ffion and Fionn in England to be honest. Surprised so many English (?) posters know how to pronounce Ffion on this thread!
I wouldn't use Fionn personally because I'm Welsh and it's awfully close in looks, if not pronunciation to Ffion. Much like I imagine an Irish/NI person would use Fionn but not Ffion for the exact same reason.
However, Ffion and my Welshness aside - I like Fionn.