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It would be good to know the size and to see views from the side and underneath. The ribs look to fine and colouration incorrect for Discus rotundatus, in my opinion.
Apologies, it is almost certainly Troculus striolatus, a species that does have tight whorls and fairly distinct ribs. I clearly had an 'grey' moment.
BMIG website: www.bmig.org.uk (centipedes, millipedes & woodlice)
BMIG Newsletters ~ www.bmig.org.uk/view/resource/bmig-newsletter
BMIG Bulletins ~ www.bmig.org.uk/view/resource/bmig-bulletin
Lat/Lng: 51.3, 0.1
OS grid ref: TQ4860