Surely it is better to have a bigger boiler in the first place incase we extend and have to add extra rad's..???
Oversizing a boiler reduces efficiency and is NOT in any way better!
Unless you had a very large house the 18 kW is likely to be totally adequate and have power in hand for any extension.
However, its always best to actually calculate the whole house heat loss so that you know what is really needed.
Tony