that's the tricky bit. I think (with the usual caveat, I am not a lawyer) that in UK law, the importers act as manufacturer's agent.But ELBC do state a "manufacturer warranty".
You can of course deal directly with the factory, send back to China (TGWG in this case) the faulty motor but I think you pay for for courier fees to their factory.
When I need to send faulty motors back to China, I pay freight there and back. They graciously pay for customs import duty (you have to declare a value when dealing with factories even when the goods have no commercial value).
It would be better for all if Bafang supply with their kits an international warranty.