Most companies will send out an investigator if the price is over 1000 quid, to have a look.
Most companies now send a cheque to the customer to cash.
Most companies will accept one quote. The customer can tell the insurance mob that they want you for the job.
All you need is to type up an invoice on your PC. Your name, address, contact number, etc.... List what needs done, what materials you need, and a price. Sign it and give it to the customer to send.
And you no not have to be registered.
Don't go mad with the price. Just price it well for yourself. Take every single little thing into account with the materials, diesel, etc....