McDonalds cannot impose a parking fine. Only a court can fine you.
Local authorities can issue fixed penalty notices for parking on public land.
Private landowners can issue a contractual demand for a penalty, if you do not comply with the conditions of parking on that land, and those conditions were clearly displayed when you parked at the site.
Look at the notice you have been given, and either (1) challenge the demand if you have grounds to do so (for example: the conditions of parking were not displayed, or (2) pay the amount demanded in the manner stated on the notice.