Watford Jn has long used pick up/set down restrictions to successfully manage this, right back to BR days.
Pick up / set down restrictions are pretty unenforceable - which is probably why they've fallen out of use in many places.
The problem with Watford is it isn't long enough for a ticket check on an 11 car train, particularly if it's busy.
Far better to have some services omit Watford - as with the other examples I gave - than create a problem "just because".