> How would I use a load balancer to fail traffic between, say, London and Amsterdam with no fiber in place between them?

What people use in those sitations is Anycast.

Of course, DNS itself, or e-mail, don’t need this kind of redundancy, since the NS (or MX) records themselves provides a list of failover servers. The corresponding alternative for HTTP, SRV records, has been consistently stonewalled by standard writers for HTTP/2, QUIC, etc.

There is an interesting draft RFC which I am keeping an eye on, but I don’t want to get my hopes up:

https://tools.ietf.org/html/draft-nygren-dnsop-svcb-httpssvc...