Most beneficial loss relief claim

That's impossible to answer and will depend on the context of the question. Simply learn the rules and apply the correct rule in the question

Yes but in the notes it says that if loss can be restricted then the most beneficial would be to bring down profits taxed at the marginal rate to the lower profit limit. Why not just use the entire loss relief to lower the total amount of tax paid

This is correct - bring a marginal company down to the lower rate 19% and then use the balance to save tax on any company paying CT at the full rate 25% - common sense