Work in progress.

Firstly don’t worry if the name is convention, con, mini-con, festival, games day or other title. Titles in the gaming space don’t tend to mean a lot.

Questions to ask yourself.

What do you want to do? Game? Play test? Shop? Talks and Seminars? Bring and buy?
Some conventions do several aspects, some are just a room of tables to play, some are trader focused with no open gaming.
For me the best thing about convention is doing the things I can do at my own events, view the traders, play the demos and do seminars and talks.

Are you a newbie / confident finding games?
Conventions provide a different amount of support, things to look for, library, player wanted signs, volunteers dedicated to help you find games, game seeker / newbie areas, dedicated games set up with teachers. (Here for Games Con has all of the above our focus is beginners and providing a great welcome) Some events or conventions are rooms of tables to game on with very little support.

Do you want to be taught games? Are you comfortable at teaching games?
Similar to the answer above, look for conventions with support, demo areas, or play areas Imagination games is great for children, Asmodee and Hachette Games UK often have large play spaces with games and teachers.

Check on total costs, food and drink, parking and hotels can push the price up.
Check total costs, new games are often not cheaper at conventions, shop around and check out the bring and buy. Most conventions don’t allow outside food.

Accessibility?
Check for accessibility pages, including access and quiet zones.

Budget a problem?
Consider volunteering directly or for larger conventions like UK Games expo companies may be after volunteers / paid demoers, this can be a great way to experience a convention

Convention and event list here https://herefor.games/knowledge/list-of-uk-conventions-and-communities/