Buying a round of shops isn't likely to happen, or if you should be able to find such a thing local to yourself you are going to be da
mn lucky!
Shops are not the be all and end all, and they are notoriously difficult to get, almost all shops will have a window cleaner, they are the most difficult of accounts get.
I know, most of my work is commercial and a great deal of it is shops, it's taken me the best part of 25 years to get there too.
If I put all the work together it would amount to about 25 hours a week.
If you are lucky enough to happen upon someone who has a round of commercial work to sell, be prepared to pay top dollar for it.
There is no reason on earth why you can't make top money doing residential work, some of my best accounts are residential.
Ignore the £3.50 brigade, they come, they go, you mentioned doing an £8.00 house in half an hour (and losing it to a £3.50 merchant) you didn't say how large the house was, so I may be doing you a disservice, but for me at least an £8.00 house would be a very small semi or a smallish bungalow (10 windows) and would take less than 10 minutes to actually clean (WFP)
you say you are in the Wirral, presumably things will be much tighter there than in many other parts of the country, but try to aim at getting as close to a £1.00 a window as you can get.
If you are going to invest in WFP, and if a lot of your work is commercial, a little back pack isn't going to be sufficient, but it would be a cheap way of introducing yourself to WFP and when needs must you could upgrade to either a trailer mount or van mount system, and a back pack will always be useful.
Price work as if you are doing it trad, and with WFP and you will do well, you'll cover your costs comfortably, start pricing cheaper just because you are faster and all you'll do is either stand still or earn less...WFP IS MORE EXPENSIVE TO INVEST IN AND TO MAINTAIN. Don't fall into the trap of dropping your prices just because you are cheaper!!!!
Best of luck in expanding your business
Ian