A 1m dish should give you BBC and ITV in your area, I think, so if that is really all you want then it should do. I’m not sure how far east Torrevieja is, the further east you go the harder it is to receive BBC and ITV. You should try and ask some local people who have Sky what dish size they have before buying one, or failing that make sure that the installer is confident that the dish is big enough. Unless you are installing it yourself, in which case you should definitely find out for sure that it is big enough before buying.
However even if a 1m dish is big enough and you have a full subscription then you are going to be missing out on many channels that you could receive with just a slight increase in dish size. Also a 1.2m would stand up to heavy rain better than a 1m, when it rains hard here ITV is one of the first channels to show signal loss, so with a smaller dish you would notice that sooner.
Did €200 include the fitting of the 80cm dish? If so how much are they charging for a 1 and 1.2m dish?
One point about your subscription, if you leave your digibox in the UK and bring your card to use in another one here, it will only work for the FTV channels and the Sky family package channels. The card will not show premium subscription channels such as the Sky Sport and Sky Movie channels, as it is tied one box only for those channels. If you want to watch them in Spain you could however phone Sky and tell them the details of your box here (pretending to be in the UK with a new box of course), but then the card wont work in the UK for premium channels and you will once again have to phone Sky to tie it back to your old box there. This isn’t actually as awkward as it sounds, but it is something to consider.
EDIT: Sorry johngra but I have just been looking at a map, you are a lot further east than I thought, what side of Torrevieja will you be using the dish? Torrevieja is getting close to Alicante and that area has big problems with ITV reception