I think we're all overthinking this. It's possible to complete this transaction the old-fashioned way.
- The transaction is a large amount. Certainly worth the cost of a plane ticket for either party.
- The buyer has already stated that he's willing to fly to London to complete the transaction.
Why not just meet him at the airport and do the business there -- in cash?
- Buyer arrives at the airport with the agreed-upon price in cash in a briefcase.
- Seller arrives at the airport with a laptop with the web site on it. And maybe a lawyer or a personal banker.
- Buyer and seller sit down in a public area (or a private suite, depending on how comfortable you are with each other) to conduct business.
- Buyer examines the web site source on the seller's computer (tether the computer to the table with one of those Kingston locks) while the seller and his lawyer inspect the briefcase to make sure the money is not counterfeit (briefcase can be similarly tethered to the buyer or the table).
- If the buyer is satisfied with what he sees on the screen, he is allowed to burn a copy of the data onto a disc right there (or alternately, let him keep the laptop), and the seller keeps the briefcase full of cash.
It sounds all cloak-and-dagger these days, but this is how my father did business all the time. There are a lot of industries (restaurants, jewelry, auto sales) where it is the norm to deal in large cash payments like this, and face-to-face meetings in public places are often the best way to make payments that everyone can be comfortable with.
I don't know about the UK, but in the United States it is perfectly legal to fly domestically and internationally with briefcases full of money. It's done by businesses all the time. But you must remember to declare the currency on your customs form.