What you charge can vary greatly depending on which part of the country you live in, but at £7.00 for a house of the size you describe is pretty low.
An experienced window cleaner, working Trad should be able to clean a property of that size in no more than 20 minutes, so you will get quicker (or should do!)
THe best way of pricing is to charge per window, as a general rule of thumb, for an average size normal casement style window look to charge £1.00 per window, although that will vary depending on where you live and work of course.
Break larger windows down to what would equate to a normal size window, ie, a standard window is 1 slim opening light, 1 fixed pane underneath it and an opening pane to the one side.
So when you look at a house you simply break everything down to a unit size and charge accordingly.
You also have to allow for ease of access, locked gates, distance you have to travel and so on, if it is a stand alone account you have to travel several miles to get to, then you have to allow for that in your pricing.
Georgian style and leaded windows you charge more for, especially if you are working with traditional tools.
Many window cleaners (myself included) have a minimum price they charge, regardless of how small the job may be, in the arera I work I charge a minimum of £12 (though there always some exceptions to that rule).
Don't price by guesstimation alone, trying to judge how long the job will take you, then thinking about how much you want to earn an hour is NOT a good way of pricing.
If you are just starting out in this game, the fact that you might knock out...say...three of those £7.00 houses you mention in an hour, that will not mean you are earning £21.00 an hour, necause you won't do it every hour of every day you work.
And as time passes you will get quicker and quicker at what you do, pricing a job on how long it takes you will can end out with you pricing cheaper and cheaper as your experience grows!!
Best of luck!
Ian