Who was the only British king to be crowned on the battlefield?

Henry VII

It is told that Henry found Richard's crown on the battlefield and placed it on his head. Henry VII was crowned king and married Edward IV's daughter, Elizabeth of York a move that was to end the Wars of the Roses.

Henry Tudor landed in Pembrokeshire in the summer of 1485 and, gathering supporters on his march through Wales, defeated Richard at the Battle of Bosworth Field. Henry became King Henry VII of England and Richard was slain during the battle by the powerful Welsh knight Rhys ap Thomas by a blow to the head from his poleaxe.

https://spartacus-educational.com/TUDhenry7.htm

Henry Tudor may have placed a crown on his own head on the battlefield, although there is little evidence he really did. But even if he did so, you can hardly call it a "coronation".

The official coronation of Henry took place in Westminster Abbey on 30th October 1485. From that moment on, he was Henry VII by the Grace of God, King of England and France and Lord of Ireland.

https://www.tudorsociety.com/30-october-1485-henry-vii-is-crowned-king/