Hi Nina,
My family always like to buy our park tickets before leaving for Walt Disney World. It saves spending unnecessary time waiting in line to do so once you arrive and allows you hit the parks straight away.
When travelling from the UK, you have two options:-
1/ Purchase Disney's Premium or Ultimate tickets from the UK version of the Walt Disney World website
HERE
2/ Purchase Magic Your Way Base tickets from the USA version of the Walt Disney World website
HERE
As to which is more economical, it really depends on how many days you are planning to visit the Disney theme parks, if you will also be visiting the Disney water parks & DisneyQuest and whether you want the flexibility of being able to park hop during your visit.
We personally prefer the UK Ultimate tickets as it means that we can visit any of the Disney parks of our choice at any time we like during our trip.
If you want to ask me another question which is more specific, I would be happy to advise you further.
Wishing you a magical holiday!