Never thought about this before as I have 12 month insurance for travelling outside UK but will check the small print. I assumed I was covered for the Camino.
Regarding your question try looking at Ramblers Association website. They probably have insurance deals or at least recommended companies.
I would suggest that you take a look at the coverage that is included with membership in the UK branch of the Austrian Alpine Club. Here is their summary:
"The policy provides OeAV members with worldwide cover, without age limit for mountain (and other leisure activity) Rescue, associated necessary but limited Medical treatment, and unlimited costs of Repatriation."
I have twice had cover while walking the camino with a U.K. company -
"Globelink" Telephone No.0870 112 8208.
From memory 6 weeks cover was circa £60/70.
Unfortunately I had to use the cover and found the service superb.