.20 was basically unknown in the U.K. until Sheridans were imported in small numbers in the early 60s.
The introduction of the 12 ft-lbs limit impeded imports of US pumpers.
In the 70s, outfits like Sussex Armoury started bringing in Sheridans modified (pretty crudely at first) to be sub-12.
Then you had the Airgunaids, which Gareth has covered.
There was an early 80s .20 fad based on the Diana/Original 45 being offered in the calibre.
Theoben had a go in the late 90s or so pushing the Rapid in .20.
.20 remains a niche enthusiast calibre, as it always has been. Id wager that most airgun owners (the ones who dont read mags or visit forums) dont even know it exists.
My take on pellet choice is that its a bit of a red herring. Yes, there are, say, a hundred (probably more) different types of pellet made in .177, but 90 of them will be cheap plinking stuff or specialised 10M match, etc. Most serious shooters will end up with a choice of a few types from JSB, AA, FTTs and such, all in standard round head form.
I count the following pellets readily available in .20: Bisley Pest Control, JSB Exact, H&N Baracuda, H&N FTT, HW FT Exact, Daystate Soveriegn and Kaiser. And you can still find the recently discontinued Crosman Premiers (including if you look in my ammo drawer: I stocked up when they were on sale ). So, one cheap hollow point (for rats?) and six very high quality brands in production and one recently out of production, at least one of which should work very well in any .20.
H&N ftt are 11.42 grain.
Which IMO is right for a .20.
Others eg JSB and Sovereign are over 13 grain and playing into the hands of those who say .20 is a pointless calibre.
Same with .25.
Again I use H&N which are around 20 grain.
Other pellets can be pushing 30 grain playing into the hands of those who say sub 12 ft lbs .25 is hopeless.
I bet Kaisers are H&N.