The Festival of Fishkeeping is the biggest UK event held every year in Hayling Island, normally over the bank holiday weekend in October.
I went last year as Practical Fishkeeping magazine's "Ask the Expert" on high-tech planted tanks.
There were loads of great discus as well as other show fish i.e koi, bettas, goldfish, various tropicals.
Festival of Fishkeeping results | Practical Fishkeeping magazine
Not so good for plants though.
The UK Discus Association is good I believe. I may join myself soon as I'm setting up a 6x2x2'.