Upon his accession to the throne of England at the Union of the Crowns in 1603, James VI and I moved his court to London. One of his first acts from London was to establish the royal postal service between London and Edinburgh, in an attempt to retain control over the Scottish Privy Council.
The Royal Mail service was first made available to the public by Charles I on 31 July 1635, with postage being paid by the recipient. The monopoly wasfarmed out to Thomas Witherings.
Info gleaned from Wikipedia