Zaphod,
I've just found this on the CAA's website
(
Guidance on using sunglasses | Medical | Safety Regulation )
"....All pilots requiring a spectacle prescription must have one clear pair of correcting lenses but can have prescription sunglasses as their second pair. The wearing of plano sunglasses on top of prescription glasses is not acceptable. For night flying, it is recommended that both pairs of prescription glasses are without tint."
Hope this helps you.
John