Here are the dates for
NGM Preschool events 2023 in Hampshire, this is possibly one of the biggest UK abdl events.
There used to be a monthly event in Birmingham during the BBB after party for abdl's and littles, but they have decided to discontinue it for a while sadly. There also used to be playspace playground parties and the ABC club in London but covid ended them, they were pretty big though.
There was a pretty decent sized event in Bristol last year that ran a couple times and plans for more but without enough background support/help they stopped (the organiser is very ill and was in hospital for at least one of the events)
There are also many many other smaller one off events through the year (I know of one in Lincoln coming up in march for littles as well as the littles retreat in Walsall and one in Wirral in April) These tend to pop up throughout the year with maybe a couple months notice rather than being pre-planned regular events, there are also a number of munches run by and for abdl's as and when people try to organise them, some are regular others less so.
So there is a fair bit in the UK if you know where to look or have enough contacts, I myself am thinking about trying to get another munch going in Bristol again I've tries before with a lot of interest but never many turning up on the day.
As for overseas, CAPcon in America is probably the biggest but good luck getting tickets as they sell out fast, there are quite a few events in Germany, Belgium and Canada too it seems but I know less about them.