Your free shares are contained in the SIP and so are your Partnership and Matching shares, if you're buying those.
You can sell your free shares once they've reached at least their 3 year anniversary, which they all have now. I think you have to sell the oldest first, but I could be wrong?
You can sell your partnership shares at any time but certain conditions apply. See your P&M booklet for more info: https://www.myroyalmail.com/partnershipandmatching