I remember just over 20 years back, I did my last job as an apprentice heating/electrical engineer in a house in Pelham Crescent. They had just paid 30k up front to a plumber..... and you guessed it he ran off like a junkie with a Giro
It was a fantastic house, 4.5 Million if I remember right. Nice people to, minted people of the old school variety
I'll take a stab and say 100 150 a day each, it seems like a lot but it's not really especially if they have enough work to cover the whole day.
How much do you earn on a good normal day? I wouldn't worry to much about what you think they will pay, charge your day rate plus travelling time etc. If you've under sold your services compared to another firm, who cares. You may find that when her pad is looking rather posh and clean her neighbours might also ask for your details.
I'd at least wear company colours if you have them and card the street.... when you leaving mind
Imagine getting another 3-4 jobs in the same street.... and doing two a day..... Nice
In my limited experience with very well off people in large properties they are not to bothered about the cost, just do a good job and you'll probably find more work.