It is very hard to say Norma, but usually large companies do not bat an eyelid at the cost, especially if it is a place with contractors. We have a local engineering company that is well known by most of the local cleaning companies, none of us will touch them for love nor money.
The blokes are a bunch or animals, the urinals, kitchen etc were appalling, we lasted on the job for a couple of monrths, and ended the contract as staff refused to work in the building.
Your hourly rate is fine, if not a tad low for the type of work that you are considering. your inintial clean price to me is a bit high, I would be charging nearer the £400 mark, but the area you live maybe more viable than mine.
Hope this help
Paul