If you are BI based and want to meetup with other in London, whether technical or business based here as some contacts.
https://sqlserverfaq.com/ – General sql server (covers lots of usergroups)
http://sqllondon.pass.org/ – London SQL Pass Chapter for SQL Server but BI people do attend as well.
https://www.meetup.com/London-PUG/ London Power BI Users Group – they meet at Skills Matter, 10 South Place near Moorgate tube, big group, lots of people attend – 50+,
https://www.meetup.com/London-Business-Analytics-Group/ – London Business Analytics Group – Very power user/Business based, less technical