BST = British Summer Time, the name for daylight-saving time in the UK (GMT) time zone. During daylight-saving this is +1 from GMT.
GMT = Greenwich Mean Time, the local time at the 0 meridian passing through Greenwich, England; it is the same everywhere. GMT remains the same all year long, but britons use BST during daylight-saving.
CET = Central European Time, is variable. During daylight-saving it is +2 from GMT and +1 from BST.

So, 8 pm BST (or 20:00 BST) is 7 pm (19:00) GMT and 9 pm (21:00) CET.