Surely you need to see the work first.
Some builders cleans are easy, some are extremely messy, and what about windows? Will you do the windows for £100 + expenses on a one builders clean.
You could be there at least a day cleaning windows scraping paint off the glass.
Find out exactly what they want first. Then quote. They will let you see the works to quote. You give a ball park figure over the telephone. But make that clear.
You will also need a UTR or CIS number, they will also dock 20%from whatever you charge.
regards
Dave