The US Embassy FBU in London has recently switched to email only. They no longer accept postal mail. [1] The FBU handles social security related matters for those out of the country.
You can visit in person only for urgent visits with an appointment which they point out are limited. [2] They no longer list a phone number of their site so appointments must be made through email as well.
Their territory reaches very far from London so in person visits for Americans in South Africa means event urgent requests are email only.
[1]https://uk.usembassy.gov/u-s-citizen-services/federal-benefi...
[2] https://uk.usembassy.gov/u-s-citizen-services/federal-benefi...