Seedy Sunday is a seed-swap event that has their 20th anniversary this weekend. 
They’ve been exchanging heritage seeds, and non-GMO seeds for the last two decades.
It’s only running in Brighton this year, without the branch events that took place at Kew Gardens, and the Seed Library in Norwood, but once Covid is past, those will be running again.
More details here,
and the current event details can be found here,