Fought August 1485 between Richard III and Henry Tudor: in our timeline the latter won, becoming Henry VII.
Points of departure:
1) Richard III wins, Henry dies - see Timeline (A Kingdom and a Horse).
2) Richard III wins, Henry defeated and leaves England - possibly to revive his claim thereafter (linking up with (a) Lambert Simnel; (b) Perkin Warbeck; (c) acting on his own account).
3) Both Richard and Henry are killed, leading to chaos as various putative candidates put themselves forward: Edward IV's daughters are captured variously several times, there is a fierce argument over whether the attainder of George, Duke of Clarence could be reversed and his children inherit. The conflict in Scotland between King James and his son is diverted into this new field of operations, leading to a dynastic marriage involving one of Edward's daughters.
4) The marriage discussions with Portugal royalty - for Richard Princess Joana and/or for Elizabeth a minor royal - are somewhat further advanced than in our time line - so Henry Tudor has to face alternative lines of succession.
5) The Borgias have a connection with England, (and are accused of being involved in the death or disappearance of the Princes in the Tower) and pursue their own interests.