Thank you!
a) Yes, I'm flying to UK and meeting all requirements to enter it, no matter if it is transit or just leisure visit.
b) According to UK immigrations rules airport change is allowed in case of "Transit without visa" concession.
transit without visa (TWOV)
c) It is not Easyjet deccision to let me into the country, your are right, but even with visa it is not guaranteed as any immigration officer can deniy entering after interview on the border.