Actually it's slightly different these days and not all RPA ... this is from the BBC guidelines which can be found at bbc.co.uk ...
The BBC is required by law to adopt a code of practice at each election to govern the participation of candidates in each constituency or electoral area. Before drawing up a code the BBC is required to "have regard to any views expressed by the Electoral Commission".
Election and referendum guidelines will be agreed by the Board of Governors and issued by the Chief Adviser Politics before each election or referendum.
On polling day the BBC, in common with other broadcasters, will cease to report campaigns from 06.00 and until the polls close. We will restrict our coverage to factual accounts, for example, of the level of the poll, politicians' appearances at polling stations and the weather to ensure that while the polls are open nothing in our output can be construed as influencing the ballot.