The excellent Wild Trout Trust begin their annual auction to raise funds for conservation work all over the UK to improve the riparian habitat for trout (and everything else that shares the water)

Details are here https://www.wildtrout.org/content/auction-0

and a copy of the current catalogue can be downloaded from https://www.wildtrout.org/assets/reports/WTT-2020-Auction-Catalogue-low.pdf

Huge range of opportunities from eye-wateringly expensive days on a chalk stream in England to very affordable opportunities throughout the UK