Here is an incredible tool which has been created using the recently released Google Maps API. It's called Google Maps Transparencies and it basically allows you to view the satellite and street views of Google Maps together. Much like a magnifying glass The Map view acting as the magnifier follows along as you drag over the satellite view to show you the streets in an overlay fashion. You can interchange the two views as well and set the transparency levels. This is a great add-on to your regular Google Maps viewing that I personally can see Google Maps integrating into a future Google Maps release. In terms of usability, this might replace the [map - satellite] toggles within the map:
Here is a bit more from the tool creator's (Kokogiak) blog:
For the time-being, it's US-centric, since their API doesn't provide a simple interface to their lovely single search box (you have to enter latitude and longitude), and to simplify things, I just included a giant drop-down of US cities. You could always zoom way out and pan to get to the UK (or Canada).
Enjoy this tool!