'Trickling' partridges out during the season is problematic for 3 reasons.
1) It goes against the code of good shooting practice
https://www.gwct.org.uk/media/768987/CodeGoodSHootingPractice.pdf which recommends that all birds be released prior to the start of the shooting season (RLP = 1 Sept) and pens removed. Any antis finding penned birds will see this as canned hunting and immediately point to a breach of the code. This is ultimately very bad for the public perception of shooting and the sport as a whole.
Even if you treat the code as purely voluntary and not your problem, there is:
2) Keeping birds for that length of time and at those densities will lead to damage and disease. You could dose them with heavy antibiotics but (apart from the costs) given the withdrawal period for those drugs, I'd not want to be eating them. Sick and injured birds won't fly well.
3) If you release birds that are totally naive to the ground and immediately shoot at them, survivors will flee the area and you'll never hold them. If you've let them out (months) earlier, they can learn the ground, establish a home base (maybe even around the release pen if you keep some call birds, feed it, shelters and low disturbance), and they'll come back there. Also makes it slightly more predictable to drive them.
So no, I'm afraid the plan is not prudent from either the perspective of your own shoot, or shooting in general. Either make sure that you've got the habitat to hold released partridges or don't bother releasing them. I know that some shoots do this, especially some of the big ones on moorland edges, even replenishing during the season. IMO that sort of behaviour will be the downfall of released bird shooting.