Yep, there isnt any part of a pizza base that can be classed as a healthy extra, its not wholemeal, and even if it were, its still unlikely to have enough fibre to be a healthy extra. Plus you can bet that the cheese would be full fat, and there's probably going to be sugars and things in the sauce - just work it on the basis of 20 cals = 1 syn and be prepared to be disappointed!
Much as Slimming World state that "no food is banned" some foods are pretty impossible to get away with and pizza is one of them!