My experiences have been poor with UK IP geolocation - so much so that I resolved to not spend more time on it until 21CN arrives (or some other major change happens).
I found that allocated IP ranges were poorly set up for geolocation to begin with in many instances (e.g. ISP or telco usting their data rather than clients, or large ranges not being properly reported/divided amongst geographically distinct nodes).
I believe that there will be better geo tools provided by someone like Google before IPs get sorted properly.
Our sites see at least one visit from around 25% of UK users per year, but that is not enough to extrapolate meaningful information (even given that people are doing local searches - there are numerous issues that are not priority 1 for us).
The things that you need to worry about include:
Has the ISP properly reported IP ranges...
If they have, have they allocated the user the correct IP range?
Is the person on a mobile device? (which we will assume you do not have permission to triangulate)
Is the user in the place that they want information for?
Is the person using a service which changes their apparent IP, or is this happeing without them knowing?
How granular (& confident) do you need to be?
It's a nightmare, but it may be that databases which are out there that do a good enough job for given circumstances - everyone will have to decide what they are willing to put up with and the build their site with those limitations in mind (possibly making it obvious how to change the locality etc).