When I used to do shop windows, most of them were in malls. The way I picked them up was to target chain stores. Don't know what chains you have in UK, but NewYorker or Just Jeans are an example of them here. Wrote to the main office with a quote, some you get, some you don't.
The smaller shops, I just did face to face with the owner or manager. It's not always abot price though, I've picked up some shops because their wc was crap or whatever.
In my opinion, there is more money in either domestic work or bigger com jobs. It's upto you I suppose, but that's how it was for me. By the way, the big chain stores want the invoice on a 3 monthly basis, so you have to wait awhile for them to pay, IF they pay on time.