Plus the old embassy was owned by the Duke of Westminster. It was leased to the USA for the grand sum of one golden peppercorn per year.
Other London properties owned by the US, were sold to facilitate the purchase & construction of the new embassy. It's far more secure and accessible than the former building which was located in Mayfair, a very congested area of London.
Bottom line is President Shithole was looking for a reason not to go to London, the Queen didn't want his ugly crass ass anywhere near the Palace, hundreds of thousands would have been out in throngs to protest President Shithole, not to mention that Prince Harry didn't want the US shithole anywhere near his upcoming wedding.