There is nothing to stop the creditor demanding larger payments unless the payments you are making are in accordance with a County Court Judgement which presumably is not the case.
If the creditor knows your present address & hence that you are not in UK he may have some difficulty in getting a CCJ in this country, but I don't think it is impossible because tyhe debt was incurred under UK law. To check this you need to contact the Courts Service.
It is also possible that the creditor could sell the debt on - maybe to an outfit in the country where you live & you might then be subject to the debt collection laws in that country.
On the whole, it is best if you can to agree increased payments and keep them up.