For the UK tickets, I think you have to use them in the calendar year in which you purchase them, at least until the following year's tickets are released at the amended prices. At the very least, they need to be activated for the first day's admission in the current year, something which would apply if you were arriving in late December and staying into the new year. This certainly was still the policy last time I checked.
On the other hand, if you buy the US Magic Your Way tickets they can be kept indefinitely and only activate on first use, at which point you have the further option to add No Expiration to them, for a fee.