Right now it's several hundred dollars to get anything electronic out of the US and into the UK, unless you're a big enough company to be able to lawyer your way out of it.
The UK has de minimis for a while yet, and PCBs are 0% import duty. The only thing you might get shafted by is Royal Mail's handling fee if it gets checked by customs. We're talking about hobbyist PCBs here, not production runs.