Documents for El Salvador need to be notarised and then apostilled at the UK Foreign Office.
The FCO has two offices. The one open to the public is at Milton Keynes, which is accessible by train from London. It is 55 miles/ 85 kilometres away. They charge £30.00 per document. Documents need normally to be signed by a notary before going to either FCO office.
The other FCO office is at Centre Point in London and is open to notaries and some other businesses. They charge £75 per document. The general public cannot go there, but notaries can take documents there on behalf of clients needing a swifter service.