I've done sub-contract work for a South Wales window cleaning company.
It was generally 'under-priced' from my point of view.
I had to provide proof of public liability insurance, and I was supplied with locally-produced forms that I had to get signed by a member of staff from the commercial property I cleaned.
Once a month I'd send these forms off to my contractors and subsequently received a cheque for my efforts.
My contractors were good; quick payers; which kept me happy, and I only stopped doing the work when I had enough, better paying work of my own.
My advice is, when you find a good sub-contractor, look after them and pay them quickly; since your work will be easily replaced.