Are there standalone PNA->Ethernet bridges available?

Yes, but they are expensive. The one I found was about $200. With the adaptors that you may need for the US->UK issue, it is probabl not worth it for the slow speed.

I'm not sure what would happen if ethernet on the outside was hit by lightning. I am running a cable like this in a similar situation, but it's fairly low on the house, and a ton of ancient trees are around that I figure will take a hit before the house does. And it is grounded from the satelite coax ground run, so hopefully it's fine.

Wireless might be the best way to go. It can cost a bit, but the ability to sit somewhere on the network with no cords is well worth it. Theres enough wireless devices that you should be able to buy two and do whatever you need (access point and bridge, etc...)