I'm just looking for some opinions here, do you think this is a useful feature to have? And do you think it should be promoted as a major feature of the site?

I don't know about in the UK, but www.pricewatch.com is effectively the premiere price search site in the US. If there already is one for the UK, it's probably not the best idea putting much effort into competing with them, unless they have an absolutely horrible set-up.