But you're dealing with completely different timezones. It's really not practical at all to serve your customers on the West coast with employees based in the UK. Not to mention the astronomical costs that would be involved with travel when needed
It won't be all jobs that can be offshored in that manner. For West Coast customers FB could easily look to offshore to Vancouver, Canada which like 150 miles north of Seattle. For East Coast customers it can look at Toronto instead of NY. London might be a satellite office to handle its EMEA operations.
Bigger point is there is talent on par with US employees that can be had at a cheaper rate. If global WFH gets pushed forward then even the well heeled developers in more creative/strategy roles might started feeling the pressure.