We've covered this in a couple of previous threads. In a nutshell, the EU courts have ruled that:
1) Copying Brompton's name, graphics, literature, or font is a violation of EU trademark law.
Curiously, multiple t-shirt/wearable/soft goods companies imprint with Brompton's name using Brompton's font and/or imprint with Brompton's graphics and that doesn't seem to bother the company.
2) Brompton's classic Andrew Ritchie trifold design is not artwork covered by copyright, but a functional industrial design covered by a patent that expired decades ago.
Brompton took both Dahon (2017) and Chedech (2019) to EU court, apparently on a specious "3D Copyright" claim. Brompton lost both times.
I think they allow it because it promotes the brand, and they're just not interested in a clothing line.