I know a number of people who fly gliders without parachutes because they are so tall they don't fit in when wearing one. Some of us only fit wearing one.
But it's not mandatory except for cloud flying - see the BGA Laws & Rules paragraph 6.14.
It is however recomended practise to wear one - see Laws & Rules RP16 & RP17.
http://www.gliding.co.uk/forms/lawsandrules.pdf