Laws and Rules (2008) states:
7.5 Before carrying a club member in a glider, the pilot in charge must be authorised by his CFI and hold a Bronze badge or higher certificate and have at least 50 hours P1 on gliders.
Beyond that it's up to the individual CFI. In theory you are qualified to take passengers, but at the very least I expect the CFI would insist you are in current flying practise for the type of launch to be undertaken. To let your passenger have the front seat (the one with the good view!) you would almost certainly need an Instructor rating.
Yes, but this assumes that you are flying from a BGA club!
What about if you buy your own glider and get towed up from your friend's farm strip?
Apart from the catch all of "endangering person or property with an aircraft" etc. I think I'm right in saying you don't need any certificate or anybody's permission.
I know that may change in the future.....