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Have you considered it being a damselfly larva that has lost it's middle 'tail' ?
It looks like the larva has 3 parts to its thorax - where its legs are attached and that is carachteristic of damselfly larvae. Also the head, with its two large eyes, look much more like damselfly than stonefly.
At first glance I can see why this could be taken for a stonefly nymph as it appears to have only 2 "tails". It is actually, as has been suggested, a damselfly nymph that has lost the middle gill at the rear. The clues are in the antennae and "tails". Damselflies have only 6 or 7 antennal segments and a one-piece lamellae making up the "tail", in Stoneflies both the antennae and "tail" are multi-segmented, with usually 10 or more segments in each.
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This is certainly a damselfly larvae (or nymph - either term can be used) for the reasons already given.
It would be preferable not to base an ID to species level on a single photograph, but the larvae of the Large Red have a fairly distinctive head shape (almost rectangular with a fairly straight rear edge), so I am reasonably certain that the photo shows this species. Typically there is also a dark 'X' mark on the caudal lamellae (when viewed from the side), but this feature can vary.
Lat/Lng: 51.407136903334, -0.082719326019287
OS grid ref: TQ334693