How can I change from Romania Store to Britain Store?

See this support article:--> Change your Apple ID country or region - Apple Support

Regards

Giulio

You need a zero account balance and you need to cancel all your subscriptions. If you have cancelled all subscriptions, but some haven’t expired, contact iTunes Support. You need a credit card issued by a bank in the country the store is in, and you need a billing address in that country. If you have an account balance, either buy something more expensive and pay the balance with a credit card or contact iTunes Support and ask them to zero balance your account.

Purchases - View, change, or cancel your subscriptions

Purchases - Subscriptions not billed through Apple 

If necessary, contact iTunes Support. Either use the form or scroll to the bottom of the page in the 2nd link. Follow through the screens to get a chat or telephone option. 

iTunes Support

iTunes Support – world wide

Do you want to change the region of your Apple ID?

Learn here how:

Change your Apple ID country or region - Apple Support

I'm sorry, what?

What are you trying to do?

If you don't see "Change Country or Region," contact Apple Support. This is a user community, we can't help you with that.

Sorry Serge, didn't see your reply in time.

Have a great day :-)

Giulio

No worries