My orange roan show type boy is 14 and was one of three in a litter of five, the other two being blue roan. It's not a rare colour by any means, but certainly a lot less common than blue roan, and less common than black and gold solid coloured Cockers I think. I don't know much about genetics, but blue roan and black are the dominant colour genes in the breed.
I'm in Hampshire too (New Forest) and see many Cockers, probably more workers than show types, in the full range of colours. Blue roans are the most common colour I see in show types and liver or black in the working types. Some of that will be due to blue roan being the most common colour overall in the breed, but I think it also partly depends on breeders in the local area.
Fashion also dictates colour. When my first Cocker puppy, a gold show type, came to me 31 years ago, chocolate/liver roans were very few and far between in show types but were becoming sought after and their numbers were on the increase. Nowadays, sables (even though not eligible for KC registration) are very popular and I wonder if that will lead to some of the more unusual colours falling out of popularity and the public eye to the point where they become rare?