In article , Helen Deborah Vecht
writes
I *think* it has something to do with where these buses run.
U* buses mostly run around the Uxbridge area
H* buses mostly cover Harrow (except H1, H2 & H3, which go round
Hampstead Garden Suburb)
E* buses have an Ealing connection
PR* run around Park Royal

I think there was once a "PB1 (?) in Potters Bar, too?
--
Ian Jelf, MITG, Birmingham, UK
Registered "Blue Badge" Tourist Guide for
London & the Heart of England
http://www.bluebadge.demon.co.uk