Having the birds on a 20% protein diet, with separate oyster shell available all the time, and few or no extras, at least for a while, will manage their diet better. Is that layer diet 16% protein, or higher? Your extras will not be as useful as having a good base diet, especially a pelleted or crumble diet, not whole grains.
Then, is whatever you are using on your birds for the ??? parasites approved for chickens in the UK? There's very little approved for use in the USA, and I hope you have looked up your product before using it!
As at least a temporary fix, the pinless peepers will keep your birds from feather picking until they can recover.
Mary