I think you have got the wrong tyres matey
I don't know if they should be the same as the mk5 golf but if so, for 18" the tyres should be 225/40/18
I am running 35 tyres on my 19's.
With the 225/35/19 it keeps the rolling measurement nearly the same as the 225/40/18.
(It doesn't throw the speedo out in other words.)
Id try and get some 225/40/18 or if poss 235/40/18, and then sell yours on.
I reckon the van would benefit from the slightly chunkier 235/40/18's