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How about these Tanaka 650b x 45mm?
https://southerndistributors.co.uk/product/tanaka-fenders-650bx45-alloy-smooth/
edit: out of stock and doesn't seem to be other availability in the UK, apologies
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Be careful with sizing for GB mudguards, because the measurement is from the external edge, and the internal roll is relatively large (2-3mm on each side).
50mm should work well, but you’ll need to cut and file a section out of each side. If you’re fixing it directly to the underside of the steerer tube, that should probably be okay, perhaps with a bit of extra reinforcement.
If that tyre is 42mm a 50mm guard will fit.