I've not seen any Marvel UK books slabbed and if they did accept them for encasing, I'm sure that they wouldn't accept them with the free gifts. I've got a feeling that they've made a statement to this effect in the past but I can't remember where I've seen it.
Actually, CGC has and does slab them and will slab them with free gifts as long as they are firmly attached and not freefloating. Therefore, if it physically still part of the book and won't damage it when it is mooshed in the slab, you're fine to send it in. (thumbs u