F-35? Volt? A pack of $50 light bulbs?
F-35 is out of bounds, as those are multi-million dollar critters. Besides, we need them.
Volt - Uh, we actaully like Great Britian. Zero has insulted them before, giving them a Volt would perhaps make them hate us worse than Afghanistan hates us now.
Pack of $50 lightbulbs - clever, and priced right for a diplomatic gift; but wrong voltage. Great Britian is at 230 V +10% /- 6%, AC, 50 Hz. I doubt they would fit into the light socket - but if they did, chances are there would be an audible pop, a flash of light and some smoke. It would say that Americans are foolish not only in paying $50 for a bulb, but that they don't last long.
If I was Cameron and forced to attend, I'd bring a toilet seat from and old slave ship. It would interest the Obama side of the family as slave traders and has Historical American value as a treasure.