b) if it's a footpath land access laws there prohibit cycling anyway
A public footpath on private land gives you a right of way on foot. It does not prohibit using a bike, but if you do without the landowners permission this would be trespassing but no criminal offence. As such the land owner would need to show damages and sue for recompense.
Just because cycling on a footpath in general is not illegal it does not prevent local bylaws from making it so which I think was probably the case in Ely and hence the signs.
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