If i did my £6 houses for £15 id earn £75 an hour. Id feel guilty taking £15 off somebody for a 15min job, regardless of 'its a service' or 'im not under selling myself'. Sometimes i feel bad charging old ladies £6! End of the day its not a hard job is it in all honesty, hardly factory work slaving away or fighting fires or saving lives. People constantly go on about under-selling yourself, id rather 'under sell myself' for £24 an hour than feel like a con man charging people twice what they earn an hour for a 15min job....
I think you should feel guilty if you were charging £15 per house and only cleaning the glass!
At the end of the day you are trying to compare your prices against other people's but from what you have said you are not doing as much work, ie. not cleaning frames and just giving sills a "wipe".
Does your pricing also include cleaning all exterior doors or do you not do them either?
Not having a go at you, but you are questioning what other people are charging but realistically you are not comparing like for like.
If customers in your area are only willing to pay £6 then perhaps you are providing just enough to suit their budget.
But I think you are being unfair believing all customers will only accept a second rate window cleaning service.
From a business perspective I don't feel you are exploiting the potential you could realise from your customers, if you upped your level of service and provided more perhaps you could earn more out of them?