UK Ordnance Survey has excellent data, and they offer a free service that allows you to embed Ordnance Survey maps, covering the whole of Great Britain, into your web applications.
I did a quick example (click it):
So how does it compare to Google Maps?
- OS OpenSpace is a lot more detailed – it basically contains every footpath in Britain. Most useful for any outdoor activity. Compare for yourself 
- Licensing: Ordnance Survey claims ownership of any derived data you create using OpenSpace . This has caused a lot of controversy, however, Google has a similar clause .
http://openspace.ordnancesurvey.co.uk/openspace/developeragreement.html, clause 5.4
 www.google.com/accounts/TOS, clause 11