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Have you used the Shark Trust's ID guide? http://www.sharktrust.org/shared/downloads/great_eggcase_hunt/eggcase_id...
The appearance looks like a S. canicula egg case but the length of the capsule (not including the horns) is 8-8.5 cm by your ruler which would put it into S. stellaris (Bull Huss) range, according to their guide. Size can be variable, though. Does it have thick ridges along the sides? I can't quite tell from the photo. The photo is good, though, and it's good to see the ruler in for scale.
To find out about what's going on in the south west check out http://www.ispot.org.uk/forum/8409
Lat/Lng: 52.4814633, -4.051972
OS grid ref: SN607890