To be honest, for a one man band with a paid for van and gear, with mostly local work, £40,000 should be over £35,000 actual income.
Ok, we can add stuff like a bike for collecting, waterproofs, charge the van at 40p or whatever it is now a mile, but most of those overheads an officevworker has with no tax concessions. E.g, fuel to and from work, smart clothes for work, a bike for the summer and so on.
Obviously, if you employ this doesn't apply.
That said, I am currently examining really closely what I want. Originally it was just a lifestyle choice, but as my circumstances have changed, I am wondering whether I want more security than a one man band will provide.