In Kidnapped, for what reason would Alan Breck Stewart be arrested if he were caught by the English Army?

The reason the British soldiers would arrest Alan Breck
Stewart involves two stories, that of English royal succession statrting with Charles I
of the House of Stuart who was beheaded in the Civil War that led to the short
Republican reign of Cromwell and that of the conflict over Protestant and Catholic
primacy in England. After Cromwell, a Protestant, died in 1658, Charles II of the House
of Stuart, a Protestant, was
called back to England from exile in France (exile due to Civil War) and restored to the
throne. He died in 1685 without producing an heir, so the throne passed in order of
succession to his brother James who reigned in Scotland as their second King James and
in England as their seventh King James, hence his title of King James II
and VII.

This
James, being Catholic in a
Protestant country, wanted to attain religious indulgence for all Catholics. He went
about this by imposing the Declaration of Indulgence and
requiring anyone serving in the King's council, Parliament or other government posts to
swear to accepting religious indulgence through the agency of three pointed questions.
The Parliament didn't approve of this and arranged an overthrow by asking James'
Protestant daughter Mary and her Protestant husband William of Orange to bring an army
and attack James. James escaped and fled into exile in
France.

We're almost at Alan Breck
Stewart. When James II and VII died in 1701 while still in exile in
France, King Louis XIV declared James' son James III and
VIII (third and eighth) King of England. James in Latin is
Jacobus. Thus the two attempts made by
James III and VIII to gain the English throne from a starting position in
Scotland are referred to as Jacobite uprisings. The second
Jacobite uprising of James of the House of Stuart was 1745.
Kidnapped is set in 1751.
Alan Breck Stewart (who is descended of kings, Stuart
kings) and his Scottish Highland chiefs fought with James III and VIII to attack England
and take the throne from Charles I of the Germanic House of Hanover to whom the crown of
England had passed. The uprisings failed, thus making all who fought for James III and
VIII, including Alan Breck Stewart, guilty of treason.
Hence, Alan had a price on his head and was on the most-wanted list of the British
soldiers. And there you have it.