I don't think that's the Richter scale. According to memory (which admittedly isn't as good as it was), the largest known (i.e. recorded and estimated) earthquake in the mainland British Isles was in Essex (Colchester area) in the 1880s, but is estimated to have measured something over 5 on the Richter scale (perhaps BL's Lincolnshire one should take that record from it).The 1930s one under the North Sea (Dogger?) was just over 6.