That could be about right, but I'm no expert in import law. I'm just speaking from my personal experience (for instance, this year my grandmother sent me a pair of designer gloves for Christmas and somehow I got charged more than the value of the gloves in customs duty/VAT!).
To save £100, I would still not choose the American option. You may have a greater selection by shopping American stores, but you're almost completely chopping yourself out of the return market. With BlueNileUK, if you get the earrings and aren't happy, you can send them back for different stones. You can also take advantage of their upgrade program in-country. By contrast, if anything goes wrong with the BG-US ones, you will be stuck paying shipping/insurance there and back all over again, risking duty/VAT twice.
For me, I'd still try and shop European. Just my opinion, though!