I have used the tiles provided by ITO World for a few short sorties to improve the names in OSM. I always use what I find on the ground when I can, but many country roads don't have name boards on display. The Ordnance Survey data that has been released often has names for these roads. I add these from the OS StreetView dataset with a tag source:name=OS_OpenData_StreetView.
I found that I could only look up OS StreetView names and ITO World's tile in an editor. I wanted to see them on a web page. I like the transparent page that sautter.com provide, so I thought I'd do the same kind of page too.
The end result (http://oscompare.raggedred.net) has either a Mapnik layer or Osmarender layer at opacity 1, with either the ITO World tiles or OS StreetView or both over the base layer. The slider changes the opacity of the overlays from 0 to 1 (hidden to fully opaque). I have only tested it on Firefox; comments are, as always, welcome.