It depends a bit on the time of day, but I'd suggest Rickmansworth
offpeak and Chorleywood peak. In both cases you can exit at j18 and join
again at j17.


Thanks for the replies - it's for this afternoon, for my brother picking
up my son on route from Northampton to Poole. I think Rickmansworth look
best as I can put him on the Met at Moorgate and the Met seems to be
running well today.

E.