Cooking pasta: when to salt?
From a chemistry perspective there's no difference between the approaches, either way you're ending up with the same amount of water and salt with the same boiling point. 

Ramsey is saying salt in first because he's adding the salt to the water before the pasta, not before heating the water. It makes sense to add the salt to the water before heating it because:


It's one less thing to do later: by the time you're ready to add the pasta to the water you might be busy, adding the salt to the water first saves you the step later
You're less likely to forget to add it: again, if you're busy later you might skip the step