Richy
As dave has pointed out, commercial work comes with more problems and stress than domestic customers, and the bigger the contract the bigger the paperwork and stress involved.
You also need to factor in the time when you will lose the contract and you will, the other month I lost 2 worth over £6k a year, that is a big hit but luckly I have enough other work to ride though it until it is replaced with new work.
But what would be the outcome had it been a £60k contract?
If you want more commercial but less stress involved, I would go down the sub contract route, the bigger comapny will have all the hassles involved, while you will have a small piece of the pie which will pay more than the domestic work you are doing, also spread the contracts out with various companys, so when you lose one its not the end of the earth.