What should I do if I have specific items/products made up of multiple materials that can’t be safely separated at site? For example, I have a plywood reel that has metal strengtheners running through it, which can’t be separated safely.

As these are not household-like materials, they aren’t included in the separate collection requirements.
If you have large amounts of waste of a particular specialist material, your waste collector may be able to find a means of recycling them, but this is not a requirement under the regulations. Your waste collector may therefore advise you put these types of items in your bag or bin for non-recyclable waste.