Suspected spies and secret wartime emissaries

How credible is the story that a wartime jewelry-shop figure in Britain was a German spy?

There is enough to justify suspicion but too little proof, because secret British diplomatic movements could create the same outward signs.

The jewelry-shop story has suggestive elements, but too little evidence for confidence that the person was a Nazi spy. Britain also conducted sensitive, unofficial contacts with other countries outside ordinary radio and telephone channels. A foreign emissary arriving by flying boat, being escorted inland, and quietly returning could look like enemy activity to local troops who had not been briefed. A family recollection about someone rowing to a supposed U-boat receives the same caution because a distant observer could have mistaken another nation's submarine for a German one. Fragmentary observations need corroboration before they become a firm espionage narrative.