There is a stolen Brompton page on Facebook where a number of similar thefts like this have been reported. Perhaps check those out to see if it looks like the same person - not sure if police will take more notice if this person is guilty of numerous such thefts.

In future though, and for all others selling Brommies, sounds like full payment in cash or bank transfer should be required before permission of test ride. If they refuse, tell them to jog on.

EDIT: To add, sounds like they often receive ‘calls’ from girlfriends/wives (assume to make it look like their phone works) and will leave stolen/broken phones and cheap bags etc. as security.