You can have loads of fun in 7 hours. Go to Windsor for the day and see the castle, you'll be there in well under an hour.

And actually, yes, the answer to your question is that it IS too complicated to get your suitcase back. Heathrow is the busiest transfer airport in the world. At any point there are tens of thousands of suitcases in the system. They are not going to spend an hour finding yours, which is probably how long it would take given that it will have been dumped in a corner somewhere before being put back into the main system in 5 hours or so.