There's no difference actually, only u pay in USD or pound. You can't apply US based sub if your SE account is registered as EU/UK(nor you can register the game if your account is in different region)
The payment will follow which FF14 region you bought.

If I remember correctly, US client will be based with US based subscription.
EU client will be based with EU based subscription.
No crossover.
However credit/debit card is not regional bounded.
EU credit/debit card can subscribe under US pay system.
US credit/debit card can also subscribe under EU pay system.
EU use "clickNcannotpay" site.
US I think utilize a different one. Although "clickNcannotpay" is global but somehow cannot link with US lol.
From there you u need buy crysta to subscribe.

They brought a nice game with a lousy system and plus a lousy-semi faulty payment system. When report complaint, they cs very good at playing taichi.
When the problem reached it limits, they'll bring down the web-chat support.
Let's hope no more taichi cs this time.

Not really needed unless you want to use Crysta... . Then you will need to add in fake address, but must pay with Ultimate Pay to ensure payment.