The arrowhead shaped leaves says it all really.
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Arum italicum has similar but larger leaves produced late in the year and over-winter, but the veins are paler than the rest of the leaf. A. italicum ssp neglectum is native in S England and S Wales and ssp italicum is widely introduced and has cream veins.
in my experience is not maculated (spotted) in many cases. I have checked with the experts, and this is almost always the right species (cicuta58 mentions others), it's just a genetic quirk.
Lat/Lng: 50.5245, -4.2155
OS grid ref: SX430717