The most he can gain is from... everything. On electronics, the prices in pounds are the same numbers as the ones in USD. So that's indeed a big gain. No VAT is another.
If he is in US and orders online, from out-of-state vendor, he doesn't have to pay tax. And if he buys on brick-and-mortar stores, he can get those tax money back when he leaves US (although that's complicated, that's the same as for US citizens leaving EU zone, they get the VAT refunded if they want).