You can upgrade to next higher plan anytime within the 12 months period you have paid for. You will need to pay only the difference between 12 months fee of both packages and that too for remaining period only.
Example:-
You pay £250 for existing VPS for 12 months, you upgrade to the next higher plan which is for £350 a year(with discount), you will need to pay only £50 to upgrade if you decide to do so after 6 months.
