There are perhaps two considerations here:
1) Your relationship with, and desire to win/retain business from, the customer. If they require full recipe dec for approval you are free to decline to provide it, but they are equally free to decide to take their business elsewhere to someone who will share 
2) Assuming this is an ingredient rather than a final retail product, you'd still be obliged to provide information to enable the final user to determine labelling requirements so there may be a necessity to share specific info on some parts (e.g. where there would be a QUID requirement, as noted above by Ian R).
One approach that I've found to be useful on many occasions is to provide approximate "bandings" for the ingredients, so you'd say that ingredient x is between 75-85%, ingredient y is 10-15% etc. rather than divulging the exact specific formulation.
It will depend a bit on what the product is though - with something like a flavouring you could be quite specific on the percentages and still not really give anything away as you'd potentially be saying something like "Natural Flavouring Substances 1.3457%", which isn't really going to help anyone to copy the formulation.
The UK market does expect a massive amount of transparency and openness that is quite alien to the rest of the world, so it does seem to periodically be necessary to come up with creative compromises to keep the customers/retailers happy whilst still protecting IP.
Edit: Near-simultaneous post with sqflady, who has proposed exactly the same approach on ingredient bandings.
Edited by pHruit, 19 July 2018 - 12:13 PM.
