So every year I can buy a new van and have no tax to pay... in therory?
No, not unless you are keeping the van you bought the year before.
I think you either need to do alot of reading or talk to an accountant before you get yourself in a financial mess.
Very true. Or ask the lads down the pub

Cant beat asking the professionals best way to go about it, how best to maximise things.
I asked my accountant similar question as to when to upgrade van. He made the point that if i spend 10k on a van, i benefit by saving 2.5 k approx (tax and national insurance savings), but as he said, ive still spent 10k out (or 7.5k then).
So it saves your tax bill, but its still money your spending.