my wifes visa begins on 12/03/08. Would this allow her to leave bangkok on 11/03/08 and to land in London 12/03/08. As im confused as to whether this date for the 12th means she cannot board a flight to the UK until that date, or basically cannot enter the UK until the 12th. Would customs in Bangkok stop her even though her flight booking shows she will land in the UK not until the 12th???

The visa acts as leave to enter the UK, so your wife may enter wef 00:00 on 12 March. In other words, it is perfectly acceptable for her to catch a flight on 11th, arriving on 12th.

Scouse.