Holyhead is generally the easiest port to get to. You'll need to transfer insurance onto the new car, and have documentation verifying you're insured on it too. You're as well off getting a proof of purchase from the seller (other than the V5) just to cover your back.

North is obviously handier if you find a car which ticks the boxes. I'd personally avoid a UK purchase at the moment given the horrendous exchange rate.