Best I can find
$ 132,466.01 Africa
$ 4,902,373.13 Asia
$ 49,423,340.29 Europe
$106,546,895.77 N.America
$ 2,509,299.46 Other
$ 6,082,217.76 Oceania
$ 944,844.22 S.AmericaConvert 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.
https://en.wikipedia.org/wiki/Wikipedia:Fundraising_statisti...
Of course, Ireland will have to move out the way. (Wait, is this an actual metaphor for Brexit?)
There are also the various territories of larger countries that aren't - French Guaiana in South America, Canary Islands are off the coast of Africa.