If I was in that situation, I would charge my normal rate and if they did not accept, no problem. They want to sub it out instead of hiring is because then they do not have to worry about managing the place either. If they hire at 7.00 then they will have to manage it and supply equipment and chemicals. If the sub it then they just have to invoice it. If you really want the work, go in just a little under what you would normally charge so that it leaves room for them to make a few quid for doing the invoicing. Just remember that if you take on a job you will be in it for a while and ties up your time for when you do get the job that would pay your rates.
Gilbert