A Peculiar Opportunity Involving Currency Arbitrage

If it sounds too good to be true...
Something about this is fishy. Even if they are old pound coins, they can be deposited to a bank account or exchanged at the post office.
I would walk away and not give it a second thought. There are all sorts of risks associated with this venture, and much hassle, for a max payout of $3500.

I don't know of any "currency exchange service" that handles or accepts coins. They're usually "notes only" - for the exact same reason as you are finding: no-one can be bothered repatriating dozens of kilograms of coins back to where they belong.

To the best of my knowledge, the only way of converting a bag of £10,000 worth of coins into British banknotes (or anybody else's banknotes) is to take them to Britain, and bank them there. This would, presumably, require you to open a British bank account since no bank in Britain is going to give a non-customer this service for free and as you point out, non-bank-connected coin changers like "Coinstars" are hard to find.

Then there's another consideration: £10,000 is above the limit for both exporting from the US, and importing into Britain, an amount of currency without reporting it to the various authorities. Since only drug dealers have a reason to go around transporting large wads of cash across international borders, expect raised eyebrows and reams of forms and paperwork at both ends, and forget about simply "not declaring" since there's no way nobody will notice all those coins in your (overweight) luggage.

Finally, there's the issue of obsolete no-longer-valid coins. "Large size" 5p, 10p and 50p coins, as well as "old-style" £1 coins, are no longer legal tender and the local coin counters in Britain are probably programmed to reject them (just like the ones in the US reject silver, Ikes, and steel cents). You can't spend these obsolete coins either since the shops will likely refuse them. Only way to get rid of those is over-the-counter into a British bank account.

Your only hope of getting "quick cash" for your coins is if you have inside help: find a British local who will more-or-less happily take your coins off your hands for you.

After sleeping on it and reading the very reasonable comments here I've decided to back-burner this opportunity.

I may travel to England for other reasons and I purchase a smaller amount in order to defray the cost of that trip.

Another get-rick-quick scheme thwarted by reality!

Metals have been quick enough for me. But past results do not guarantee future returns. This scheme guarantees an immediate -100% return.

If they are old pound coins (pre-2016) it's very hard to pay them in anywhere - only the post office, not banks, will accept them.

I sent all my old pound rounds (400) to leftover currency (the website). They paid 1USD per coin and I bought them to 0.50. Shipping was 50 bucks to it was sort of worth it.