In the U.S., copyrighting a design merely means you can't reproduce the design documents. It has no effect on the design of an actual product (which would be covered by a design patent, which has a lifetime of 15 years).

(Brompton is a UK company, so this wouldn't apply to UK-based competitors, but it would protect US-based competitors.)

EDIT: it looks like the EU actually has similar rules. Brompton is very likely going to lose this case, and is probably just counting on the court proceedings to delay this competitor's entry into the market. Expect a flood of Brompton-likes when the lawsuit settles.