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Price also doesn't determine the quality of a food. When I have to feed kibble my JRT is fed an inexpensive, but good quality food. Many vet pushed or grocery store brands are far more expensive, but have inferior quality ingredients. It's the marketing that makes them expensive, not the quality of the ingredients. There are many choices for consumers, but people first must have the knowledge to make the right choices.
I'd be interested to know which "inexpensive, but good quality" kibble you feed your JRT - problem is there are a lot of us out there (me included!) that don't actually know what is good quality and what isn't 
He's fed Chicken Soup for the Dog Lover's Soul, which I pay $1.10/pound at a local store. Online it usually goes for a few cents more. I don't know if that particular brand is available in the UK. I mostly like it because the first four ingredients are high quality named meat products, but there are many other reasons why I would consider this a good brand, which I don't really want to go on in length about. In comparison, Hills Science Diet, which is primarily corn and other poor quality protein sources and fillers usually markets at least 60 cents more for the pound. When bought at the vet they are probably priced much higher. My other dogs are feed a premium grain-free kibble, but my JRT is an older dog who spent most of his life eating garbage which is why I think he can't handle rich foods.
I base quality strictly on the ingredients listed, not on paid advertisements or how well someone else's dog has done on a particular brand. Searching through past posts on this forum or doing a quick search online will provide you with the information to understand what are good ingredients versus what is poor in quality and may be even harmful to your dog in the long run.
