Apart from the VAT issue, many Defenders are registered as commercial, and this can have significant implications.
For instance, do you use any toll bridges or roads? If so you will often pay a higher price, eg £13 instead of £6.50 on the Severn Bridge.
Your insurance company may want to try and charge you extra.
Some camp sites will not allow commercial vehicles.
It doesn't matter that it LOOKS like a domestic vehicle - it's what is on the V5 that matters, and I have been told that it is no longer possible to change this.