I find AA routemap always wants to take me to the reigate M25 juction for almost any journey even though Caterham or Badgers Mount are normally far easier. Putting a start point in part way down the route gets round this.
M25, M20, A20 - It's a doddle. I went that way earlier this year when 4 of us did a Calais CR. Nice quick run down although boring motorway driving all the way.
Easy route as described is the one I use, but be careful coming back to stay on M20 (keep in right hand lane at junction) and not get sucked along M26 otherwise have to go all the way to Caterham.