Interesting thoughts guys. I also found the 'wrap' one rather obvious.
And all chocolate brand name puns have been done to death. I would rather the punchline properly related to hospitals or the NHS. Maybe a pun of 'slab' would be better.
When I sometimes say 'I can't think of anything' what I really mean is 'I can only think of lots of really obvious stuff that I wouldn't dare submit and seriously expect it to get chosen.' But it seems a lot of people don't have that filter (and still get chosen anyway).
All that said, I'm quite new to this and someone who's been writing jokes for a while would probably think my jokes are weak and hack. So I shouldn't speak to soon. :-P