The issue is not ETRTO349 or ETRTO355, there is only 6mm difference !

The issue is the tire clearance on the Brompton frame that limit the tire width to about 37mm (no space in tire width and tire height to go beyond 37mm). .

If there was more space, asking Schwalbe to make for instance a Big Apple 50x349 or Continental to make an Contact Urban 50x349 is not a problem.