More likely for a light. Sometimes they had a tab also for mounting the light since it's only on one leg. If it was for a pannier then mostly likely both legs would have mounts. I don't think British bikes got too much into panniers like French builders/riders. With the mount on the fork and the pump pegs it probably wasn't a race bike or any sort. If it's a bolt then it most likely used this style of light mount. Is it on the right fork blade?