It is possible to use the Ordnance Survey Open Data 1 to 250,000 raster maps as a background layer in JOSM. However, the positional accuracy of this scale of map is limited. This could also apply to the StreetView tiles, but they are readily available as a WMS layer, which is probably preferable.
OS OpenData
OpenStreetMap editing with JOSM
I have done most of my Open Street Map editing using the free JOSM Editor, as I feel it has a number of advantages over the default, Flash-based, Potlatch editor built into the website. The feature I use most is the various background image layers, but also being able to quickly loading my GPS tracks and support for other plugins are useful too.