Looking at traffic movement today, keeping on the south circular to Kew Bridge may prove to be the best option and once across the bridge take that shortcut up Lionel Road South (it is poorly sign-posted and looks like a back-road but is very effective) that brings you directly onto a slip-road for the M4.

Watch out however if you miss that little turn-off onto Lionel Road South as there appears to be a 45 minute delay at the next main roundabout.

You are right that the M25 + M3/M4 triangle might avoid all problems.