An apostille is a verification stamp and is required if a document to be used abroad. It confirms that document has been signed/certified by a legitimate professional such as a notary public, solicitor, registrar, court clerk, GP. Full list of persons whose signature could be confirmed by apostille is on the official UK government website. Some official documents such as Memorandum and Articles of Association, Tax Returns, GCSE and A-level certificates, Bachelor and Master certificates, Certificates of Naturalisation or Certificates of No Criminal Records cannot be apostilled without prior certification by a notary. Also, instead of certifying an original for the legalisation purpose, you can ask a notary to execute a true copy of the original and apostille it instead. In fact, it will be preferrable option as in this case you can apostille a few documents together under notarial certificate and will need only one apostille. If documents legalised separately, each of them much bare apostille stamp.
Each apostille has the unique number and any official or individual can check the apostille on the official government website www.verifyapostille.service.gov.uk