Pricing is a very personal thing to each individual business.
First work out what your expenses will be a year are then work out how much profit you would like to make. These figures will determine how much you need to earn to just stay in business. Then work out how that fits in to an average job you do.
For example: If you feel you can do two jobs a day minimum, and you were to charge your minimum rate at £40, that would be £80 a day, at 5 days a week and 46 weeks a year that would bring in £18400 a year. In an absolute worst case scenario if you can run a business on that and survive, that is your minimum charge should be. The reality is if you take all your expenses off that though it's not really a great amount of profit, if any, therefore your minimum charge should reflect what you really need to make to stay in business and live.
As what to charge per room.. how long is a piece of string. Is it a single or double bedroom? is furniture being moved or not? Is it heavily soiled or lightly soiled? Is it wool or poly prop? Is the access good or is it bad? Are the customers easy to deal with or difficult? Are there stains? What are the nature of the stains? As there are so many variables it is impossible to have a set price per room. A price per square ft or meter for each situation will solve that issue, of course that means visiting the each job to give a quotation beforehand, and that needs to be priced in too. Totally hypothetically say an average lounge takes on average 1 hour to clean and the average lounge is 10ft by 16ft that would make it 160ft and lets say for the average lounge you would like to make £70, that would mean for an average lounge you need to charge 43p per sqft. ( i have just made these figures up ans an example they are not real)
It is best to start off as you mean to go on. Let your yearly out goings and the amount of profit you wish to make dictate your prices. Rather than try and compete on price be determined to compete on quality and service and you will be able to command a higher price, do less work a day, make more money and keep the customers.
All the best in the new venture.