A short(ish) answer is - generally a politician is not paid for their contribution to a TV or radio programme if they are appearing to express their policitical views etc. However, if they are appearing in a capacity outside their normal duties, then they may be paid a fee so long as it does not exceed a fee that another individual appearing to give a similar contribution could expect. Have a look at the BBC's Producer's Guidelines - bbc.co.u/info/editorial/prodg/chapter33.shtml, for an example of guidelines.