You're quite right, it doesn't look English. Plus, I've never heard of simply marking a spoon with the London assay mark and nothing else. What's more, this has a few differences to the crowned leopard, most notably the cartouche.
However, I've not heard of any other country using such a mark; very strange. The design looks Dutch to me.
Miles
PS Is that a mark to the top right of the leopard?

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