Since it's not unusual for my relatives to fly between the US and UK with snacks and even Halloween candy and processed breakfast cereal, been there and done this with these exact items at times. The UK -- at least at LHR -- has a lane after baggage claim where you can try to declare them; but UK customs will generally waive people by who declare candy and crackers. And wheat thins and Reeses have elicited a sort of "get out of here please" gesture/look.
With dealing with US CBP, they are to be declared each and every time. That is even as they will most often not send you to Ag inspection for this. And even Ag control will usually not care and wonder why you were sent over for Wheat Thins and Reeses.
Both countries will commonly let this in and out without issue, but US CBP is way more sensitive to food declaration accuracy than the UK border force when it comes to this stuff coming in and out at airports in personal quantities.