You may either just turn up or, to secure a place, may book via Eventbrite (for free): details at llgd.org.uk
There is a transport strike on the day affecting trains nationwide. You can, however, still get into the City. It is believed that the following are running:
Croydon Tramlink (doesn’t go to City but might help make a connection)
Docklands Light Railway
Elizabeth Line (some disruption)
London Underground (some disruption; Richmond and Wimbledon District line may be affected as these are part run by National Rail; also Wimbledon station is closed for engineering works)
Southern Railway (limited service, check on the day).
There may be a skeleton rail service by other rail companies. Bus services unaffected.