Who is authorised to legalise documents?
The Foreign, Commonwealth and Development Office (FCDO) is the only government entity authorised to legalise documents – and issue apostilles in the UK. 
They confirm that the official UK document is authentic and add a seal to the documents to prove the signature on the documents issued in the UK are genuine and ready for international use.
The legalisation office in the UK provides apostille services to countries not part of the Hague Convention through the FCDO as required by the Hague Convention of 5 October 1961. 
Do I need my documents legalised?
Yes and No. The legalisation requirement will depend on the country in which you wish to use the documents and the specific UK document at hand. Some documents do not require legalisation.
Documents that will require an apostille from a public official include degree certificates, birth certificates, custom letters, court documents, etc.
If you do not intend to use your document outside the UK, then you don’t need to have the document legalised.
Also, if the document is already certified, you will not require any further legalisation. This includes documents such as adoption papers, civil partnerships, etc.; these are documents that the UK General Register Office has provided. 
What is document legalisation under the Apostille Convention?
Document legalisation under the apostille convention means that; If you are planning on using your document in a country that is a member of the apostille convention, you do not need to go through the legalisation process.
All you need is one stamp to authenticate your documents, showing that the stamps and signatures are genuine. An apostille certificate is attached to your documents, and they are eligible for use in your receiving country.
How does legalisation work?
The legalisation process takes specific steps that work towards confirming the authenticity of your original documents.
Usually, two countries are involved;
Country A, where the documents originate or are coming from, and the receiving country, B, where the documents will be used.
Proof of authenticity by issuing an apostille by country A
Once the authorised body in country A proves that the documents are authentic, they provide their stamp of approval.
After approval, a stamp or sticker is placed on the document. This shows country B’s authorities that a competent authority in country A has issued the document.
For countries in the apostille convention, the legalisation process up to this point is adequate, and you do not need further verification.
For other countries not bound by the apostille convention, their embassy, honorary consulate, or the consulate general in the issuing country will take a second look at the documents and attach a sticker or stamp to the document to prove its authenticity.