Well, I compiled the UK data into CartoType format and this was the result:

06/04/2008 00:06 26,632,558 uk-080123.ctm1
26/01/2008 18:45 696,113,272 uk-080123.osm

That is, the CartoType CTM1 file is under 27MB for the whole of Britain. It seems to work fine using the Windows viewer. Tomorrow if I have time I'll load it on to the Windows Mobile viewer and see if it works acceptably fast.