[..]
Oh, and it goes without saying that as a bloke I can't admit to trying to watch my weight.
No, it doesn't. If you want to make it more masculine, dress it up as improved fitness rather than reduced weight (which is a much more sensible attitude anyway).
My suggestion is to tell the truth. If you're determined to lie about it, sooner or later someone will see it as rude whichever lie you choose. If the cake offer is once in a blue moon, you'll probably be fine. If it's fairly common, you'll probably offend someone.
Your lies have their own advantages and disadvantages:
1) Refusing the cakes means that you have to speak to them more. They might push you. You'll have to say "no" more clearly and since you're lying about the reason for it there's plenty of scope for offending people.
2) Accepting the cakes and throwing them away later will probably be seen as more offensive if they find out what you've done. It's also possible that it will delay problems rather than remove them - what if they ask what you thought of the cakes? If the cakes are home made, that's very likely.
Tell them the truth. It's not something weird, like that you're going to take the cakes home and masturbate with them. You're just being a bit more moderate about what you eat, for sound health reasons.