This question would be impossible to answer without looking at the shop. Depending on how big the windows are etc and how long it will take you to do the job. All these factors will give you your final price.
The £1 per window will be a good guide for you. But you really have to build a pricing structure.
Work out how much you would like to earn, expected running costs. Then work out how many hours you think you'll work in the one year, take some time off days were you will get rained off, high winds etc, count in a little down time for being off the road things like that. Then divide what you need to earn by your hours and that will give you a rough hourly rate that you should be aim for.
Ours in £15 per hour, but that is like trying to guess how long a piece of string is on here, everyone will be different, different areas, different levels of service etc.
Next your really have to get a little experience and work out how long jobs take, then build a pricing structure that will match your targeted hourly rate. If 50p for a small window £1 medium, £2 for a big window, gets you that hourly rate, then go with that. Else come up with some other kind of system that works.
Pricing is one of the most difficult things to do when you first start up. Its ok saying that you should base it on how long you think the job will take to do, but when you are a novice, you dont know how fast you will get in a few months time. I have had a few properties where, initially, I thought I had underpriced, but now that I am speeding up (slightly!!), the pricing for these same houses seems more accurate. I have been at it now, full time, for 4 months and still do not know how fast I will get. I met a local windy today who has been in the trade for 9 years and he can do 3 houses for every one that I can do and we are priced similarly...just hope I can get up to his speed without sacrificing quality. Suppose its all about gaining experience and being patient.