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Try a gallery https://www.hatfieldhines.com/artists/henry-bird/ or https://www.hargravefineart.co.uk/henry-bird/ they might have clients.
Failing that, if you really want to sell it then try one of these auction houses https://www.invaluable.com/artist/bird-henry-richard-oycesu03sl/sold-at-auction-prices/
Not sure you'll be able to retire on the proceeds, so might not be worth the effort. Complete paintings don't seem to go for over £200
I have an unfinished painting by somewhat well-known artist Henry Bird. However I don't like it. Where and how should I attempt to sell it?