Snake?

That looks like a Slowworm to me, technically a legless lizard rather than a snake. A great spot though, was it in the pond?

Agreed, that looks like a slow worm. Not something you see swimming very often.
If you see a snake swimming in the UK it is most likely to be a grass snake (which would have an obvious black and yellow collar). Adder (which have a zigzag pattern) swim well, but less often. Smooth snakes are restricted to a few heaths in central southern England (Dorset/Hampshire/Surrey) and are not found elsewhere in the UK.
Slow worms are actually lizards, not snakes, but are very widespread and often occur in gardens (as do grass snakes) so can be quite easy to find - if you know where to look

Bit late to comment on this, but I'm pretty sure that's an Adder, very rare to see swimming - you can tell it's an adder by the very distinctive head shape, and dorsal crossbars / patterns on it's back is just slightly blurred by the slow shutter speed of your camera / phone.

www.nhm.ac.uk/.../slow-worms-most-unusual-lizards-in-britain.html.

This popped up in my inbox today. On first glance I thought slow worm, but I actually agree that it could be a young female adder. The head shape is much more adder than slow worm, and then there's the arrow pattern on the head. Agree the rest of the body pattern isn't up to much.
www.alamy.com/a-young-adder-common-european-viper-moda-mija-zygzakowata-image381582139.html

I can see why you might think Adder as it has triangular markings on the flanks, but these are not zigzags across the whole body as an Adder would have. This is a Slow Worm, as our reptile expert Whistling Joe has already said

I zoomed in so you can see the head properly, as I said the zigzags are not clear because of the motion blur on the camera...but the head confirms it for me.