I have extensive vitiligo. Where can I purchase monobenzone, in the UK.

Vitiligo is a depigmentation in skin mean there is no pigmentation in vitiligo spots. Whereas other parts except vitiligo have some pigmentation in these.
Mono cause the pigmentation loss in your pigmented part of skin and hence discoloration in these parts appear consequently these also look white and difference between vitiligo and other pigmented part vanishes.

You should visit your GP who will refer you to a dermatologist. He/she will most probably prescribe it for you.