Certain doesn't have a separate BST time zone. For events in the UK, pick (UTC+01:00) Greenwich Mean Time : Dublin, Edinburgh, Lisbon, London under Plan > Event Setup > Details, in Time Zone. The offset follows British Summer Time, so your times are right. Only the name says Greenwich Mean Time.

Where the GMT label shows up

On your form pages and in your emails, the time zone comes from the Event TimeZone data field. It shows the full name from the list, for example "(UTC+01:00) Greenwich Mean Time : Dublin, Edinburgh, Lisbon, London". That text can't be changed.

Show BST instead

Replace the data field with your own text wherever you want BST.

1. Find where the time zone shows. Check your registration form pages and your emails, such as the confirmation and reminder emails.
2. Edit the text. Open that page text or email and delete the Event TimeZone data field.
3. Type the label you want. For example, "BST". Then save.
4. Test it. Preview the form and send yourself a test email to check the label.

Add to calendar

The Add to calendar link on the confirmation page sends the London time zone with the event, so people's calendars show it at the right local time. You don't need to change anything there.

Next step: List every page and email that shows the time zone before you start, so you don't miss one.