It's like any animal product - you'd have to declare it - better safe than sorry.
However, if they're anything like New Zealand - who are VERY strict, they can fumigate / sterilise / wash any suspect item. For example, a friend brought an Ostrich egg back from South Africa. They took it for a couple of weeks, processed it, and mailed it back to him, all customs-approved.
Similarly, bugs that are dead and pinned can be fumigated to kill any eggs, and wood carvings can be dipped/sprayed as well.
Gatwick won't care - you're exiting with it, but you'll need to declare it at the US point. Odds are they'll wave you through, or possibly just inspect it.
As long as you declare it, you won't get fined. It's if you're caught trying to sneak something through without telling them that you'll get into trouble.