I think you'll get it down to 8hrs deffo. How did you clean the leaded?
I honestly think this is a mistake. Assuming that you can get the job done in less time, while it is reasonable that this will happen, you may also be in a position that they only call every 6 months. It then becomes an initial clean each time. The very fact that others are quoting £200 for just the outside should sound the
alarm bells as you are doing inside and out for this price.
One of my greatest mistakes when I set up in the last recession was to under price. I thought I had to go in cheap to get the job. The problem is you are stuck with the price and any increase will be difficult if you ever want to be in-line with the other window cleaners. They charge that amount for a reason!
While you will get the work, however from my experience, you can get stuck with hard cheap work for some time. This type of work should be the cream trying to "get it down to 8 hours" will turn it into a chore.