HM Revenue & Customs: Home Page
This is a good place to start.
Get in touch with a tax inspector and get a two way communication going. Believe it or not, they are actually very helpful and will suggest all the allowances and expense claims that you are entitled to.
If you go this route, at least you are always going to be legal!
Don't evade as you know very well that you have to pay tax somehwere.