I have a UK account I'm going to use, but my CC's are US-based. Will there be any raised flags?
I'm really hoping I don't run into your "suspcious activity.."
I would say yeah they will, if it does happen you could try and say you was on holiday or something which might explain it and hope they give it you back.
Where is your ip based?
Well IP is based on NY, but not sure how I can resolve this CC issue to match UK.
Use HideMyAss VPN for New York IP. They don't care if you have marked your account as UK. But logging in from uk and then from ny will get your account to verify your friends on photos + maybe security question 
My old facebook account got nailed with suspicious activity after I'd been in Canada for about 2 months with my Australian CC.
'Tis an agency acc