Maybe creating an application with a list of UK exchange prices? Some that calculate the possible profits from buying from one and selling to another. You could compare the prices with other exchanges around the world to tell the users how much they can profit from buying outside and selling locally, etc.
As most exchanges already have open APIs for this kind of thing, I don't see it as a big of a deal to build.
It wouldn't be difficult to build, but I doubt that you'd get any decent amount of money out of that.
You're catering to a very specific localized subsection of Bitcoin users and have no clear way of making money.
I doubt you could charge anything for a service like that and other forms of revenue are limited when you're serving such a small niche audience.
To OP:
An exchange might be a good idea, if you have enough capital for that.
Look at this example, this is a Dutch/Belgian exchange that charges a premium for buying and selling Bitcoins instantly. They even have an option to send the payment to your bank account within 1 day (And having the money available to spend the same day)
It's this:
http://www.bitonic.nl This exchange is purely geared towards end users of Bitcoin, no option of really trading professionally on their website. And they have a pretty high volume.
I'm not sure if there's a similar service like that in the UK, if not, starting something similar might be a pretty lucrative endeavour.
