A 18th or 19th century kosher food seal, known as a plumba (ie “lead”). The two ends would be clamped onto a food item to indicate that it was Kosher. The larger end here is fairly generic and has two sets of three Hebrew letters: אבד (aleph, bet, dalet) and דככ (dalet, kuf, kuf). These stand for “av bet din” and “kehillah kedosha” giving “Chief of the (Rabbinical) Court of the Holy Community”.
The other end of the seal would identify the supplier who had prepared the food. In this case only a N is visible. Given the find location it is possibly from the London meat markets.