It looks from another chart like 21% of revenue is recurring, so $10 million annually for Europe.
Convert that to GBP and you get about £7.5 million.
Figure that the UK accounts for about 15% of the European economy and assume it contributes to Wikipedia at about an equal share.
That's about £1.125 million in estimated annual recurring contributions from the UK.
You said 10-100/mo, so let's assume £55/mo or £660/year as the mean.
That would be about 1,700 Brits, a surprising number.