Furtehr to Aligator's comments about the crosses in
Cornwall, a lot of these were 'Celtic' in origin and in use
throughout from that time.
if no literature is available from this list, I suggest that
you contact An Liveras Kernewek (sp?) in Meneage
Street, Helston, the Cornich Bookshop who will know of
any literature on the subject. I believe that I saw
something in there about crosses last summer.
Failing that, Cornwall County Archaeology in Truro?
Afraid I have no information on the rest of the country,
Also, of course, try the museums in Truro and Penzance.