I'd stay relaxed. The more as you can't do anything now. The bike has been registred at uk bikeregister (very common in the uk) as one can see from the red and white sticky on the frame below the saddle clamp. So in case it was stolen you can find it out quickly and should contact them anyway for registering it to your name. Other than that Brompton offers a stolen bike finder as well. Again I'd recommend either registering it at Brompton or - in case it has already been registered - to let the registration being rewritten to you. Might be a bit of a hassle as you possibly did not buy it from the original owner but worth it.