The three Scottish ones where you will find the most unaccompanied singing are as follows:

Fifesing Traditional Singing Festival - May 15th-17th 2020 - based in Freuchie and Falkland (in Fife, of course!)
Fifesing Traditional Singing Festival

Cullerlie Traditional Singing Weekend: July 24th-26th 2020 - at Cullerlie Farm park - a few miles west of Aberdeen. Apart from teh singing there is a chance to learn rural crafts!
No other details of this year's event yet, but worth bookmarking the website:
Cullerlie Traditional Singing Weekend

Then there's Falkland Traditional Music Festival (also in Falkland, Fife, which took a break last year, but is up and running again this year: it has gone back from June to August to the same weekend that used to be the one for the long-running Auchtermuchty Festival, 7th-9th August:
Falkland Traditional Music Festival

And if you like a bit of Scottish history, no better place to be than in Falkland: see the Palace, including Real tennis court where Maty Queen of Scots might have played!