Ofcom has published detailed maps of Britain’s 3G mobile data coverage – and they make grim viewing for owners of the iPhone and the forthcoming Palm Pre.

Both Apple and Palm have decided to lock their devices exclusively to the O2 network. But as Ofcom’s network-by-network maps show, O2 has by far the patchiest 3G coverage of any of the UK’s five mobile networks.

As we can see from the O2 coverage map shown above, vast swathes of Wales, Scotland, the north of England, Norfolk and Cornwall have no 3G coverage whatsoever. Even parts of London and the Home Counties are not covered.

In typical Ofcom fashion, the regulator doesn’t even have the confidence to back its own data, stating clearly that the January 2009 map data “was not collected for the purpose of examining detailed mobile coverage therefore the availability or quality of mobile services cannot be assumed from these maps”.

So I visited O2’s own website and decided to inspect the company’s own HSDPSA coverage map (shown below right). It paints a remarkably similar picture, with Wales, Scotland, East Anglia and the South West virtually bereft of 3G coverage.

The iPhone requires a decent 3G connection to take full advantage of key features such as the App Store, full web browsing and downloading maps – the very same features that O2 and Apple trumpet in their glossy TV adverts. So why are Apple and Palm hitching their wagon to a network that patently can’t deliver such services to large parts of the UK?

If Ofcom needs a good reason to ban phone manufacturers from signing exclusive deals with mobile phone networks, it’s just found one.