Ringing Forums

I think your best chance of getting local advice is to talk to the district ringing master for the relevant part of Essex (and Herts) as they will probably know the level of ringing at the various practices. Most Association websites give you practices by practice night so if Thursday is the specific aim that will narrow it down.
It’s also worthwhile to talk to the tower captain about your concerns and try to find out why he is focused on Grandsire triples, for example.
Some tower captains focus more on the ringers who ring with them on Sundays and see others who join the practice as ‘visitors’ who are there to support more than to gain.
In general improving your ability to plain hunt on 9 or 11 is helpful towards an end goal of ringing Cambridge because it requires you to place your bell precisely so don’t discount this kind of ringing as good for your overall skill set. But if it is a struggle for too many of the band I can see how that makes it hard to benefit from it.
A practice which has enough ringers to try plain hunt on 11 might also mean the tower Captain worries about the number of people sitting out when practicing minor. It could be a useful conversation to see if they are aware of people’s ringing goals and whether others in the band would also benefit from more ringing of minor and therefore not mind having less total ringing time if it is more targeted ringing time.

It sounds as if the practice night has become a victim of its own success! If this is not your home tower, you don't have to keep attending.

I agree with Lucy, even without an opportunity to ring Cambridge, you can still be improving your skills. I found in that "aiming for Cambridge" phase, the most valuable rope time was a local "Plain Bob doubles and minor only" practice which I attended as a helper. It helped to embed the dodging positions properly and made the bobs and singles completely familiar.