But if I send that to someone who is in a different timezone, e.g. in America/New_York, will it put the event at 6pm London Time in their calendar, so that they can videocall in at whatever time that is where they are?
The ical format allows dates like DTSTART;TZID=Europe/London:20230901T201000
Whether your calendar software will let you choose how it expresses the timezone and whether the recipient's calendar software will store it that way is another question...
Google Calendar does, at least. It even supports representing events that being and end in different timezones (like a plane flight or a cross country trip)
