Legalisation is the process by which the signature and seal of the Notary are authenticated by the Foreign Office and/or the Embassy or Consulate of the country in which the document is to be used. This is generally required by countries who are signatories to the Hague Convention. Legalisation requirements are determined only by the country where the document is going, not by the UK government.
Documents going to countries which are, or have been, part of the British Commonwealth seldom need legalisation nor, at present, do documents going to many parts of the United States.
Many countries require only one certificate from the Foreign Office called an Apostille and this costs £30.00 per document. Your Notary may charge an additional fee for dealing with this aspect.
The Foreign Office is located in Milton Keynes (see below) but a Premium Service for business users only with a 24-hour turnaround is available in London at a cost of £75 per document (plus courier fees).
Some countries want the Apostille from the Foreign Office and a further certificate from their own Embassy. This will take longer. There are other procedures that apply only to a few countries, and there can be problems if a document is to be used in a country with which the United Kingdom does not have diplomatic relations.
Your Notary may not be able to tell you in advance what legalisation will cost, but fees can be high for some commercial documents.
The Foreign Office address is The Legalisation Office, The Foreign Commonwealth and Development Office, PO Box 7656 Milton Keynes MK11 9NS. They have a useful website at: https://www.gov.uk/get-document-legalised
You can also check whether a country is a member of the Hague Convention athttp://www.hcch.net/index_en.php?act=states.listing