I would stay away from decathlon if you are really keen and intend to use it allot.
Spending a bit more will mean you only have to buy once.
Have a look on Magic seaweed (they have a surf forum as well) often have good sales.
If you are using it on the coast pay a visit to a specialist shop and try some on. Fit is the really important thing as tons of water sloshing around wont keep you warm nor will to tight and a good shop will help you with this.
Also check second skin and snug if you want to buy British. You can also get there suits repaired instead of having to buy a new one.