River Thames? As far as I am aware, the M25 becomes A282 either side of the river. I guess this is to enable learner drivers (who normally cannot drive on motorways) to use the Queen Elizabeth Bridge or Dartford Tunnel.
There is nothing special about Junction 1 - if you are driving clockwise around the M25 you have to use junction 1a, the A206 - there is no exit clockwise at Junction 1b.
The actual crossing/bridge has to be designated an A road otherwise traffic that is banned from motorways could not use it to cross the River Thames.
You have to cross the Thames on the A282 to get to junction1a. There is no junction 1.
Not a lot of people realise that the M25 is not a complete circle.