The RAF have quotas to fill. If they need one thing more than another, naturally they'll try and get you to change your mind.
Dont let them pursuade you with the story of how its easier to swap branches once you're in, because it's not!. If you want something that bad, you won't change your mind that easilly, and being determined enough to work for it is part of what they're after.
Have a couple of options, but if you're being offered something you really don't want, don't be stupid enough to accept it. Air Force life is great, as im sure most will tell you, but if its a branch you dont want, then its just another 15 years of unhappy work, when you could be on Civvie Street doing something more appealing to you.