You wouldn't be allowed to use transitional glasses, due to entering tunnels etc and them not changing quick enough. The official quote from the RSSB is:
Correction types
B.2.4.1 The following types of lenses are in general use for spectacles:
a) Single vision distance.
b) Single vision near.
c) Bifocal.
d) Plano-reading bifocal.
e) Progressive (varifocal).
B.2.4.2 Any of the above lens types can be obtained as photochromic (photochromatic)
lenses, designed to darken as light levels increase and which are an alternative
to clip-on sunglasses over clear spectacles or to prescription sunglasses.
Photochromic lenses are not permitted for drivers licensed under TDLCR. They
may adjust to low light conditions unpredictably and have an unforeseen effect on
drivers visual acuity when moving from bright light to dim light, for example,
when entering a tunnel after being in bright sunlight. In addition, photochromic
lenses rely on ultraviolet light for their action and modern windscreen glass may
exclude ultraviolet light, rendering them ineffective as sunglasses.
For more info look on:
http://www.rgsonline.co.uk/Railway_...nt/Railway Group Standards/GORT3451 Iss 4.pdf
Hope this helps.