Seems that Chelmsford and Brooklands are diagonally opposite each other, so a loop is possible without any extra effort.

The downside is that I may decide not to do this due to idiots throwing concrete blocks off the bridges on the A12:
BBC NEWS | UK | England | Essex | Concrete blocks thrown onto cars

This was a problem at the beginning of the year, but it seemed to have stopped. If I go another way to the M25, I’ll have to forego the circuit. Bah!