The place of dead kings - Geoffrey Wilson

It is 1855. The English revolt has failed, and brutal General Vadula governs England now. Only a few small bands of English rebels still hold out against the Rajthanan empire.
Jack Casey survives in remote Shropshire, training young rebels to use the conqueror's magic. But he is gravely ill, with only two months to live . . .

Then refugees bring with them news of a rogue Indian sorcerer in Scotland. Mahajan has discovered a mysterious power in the uncharted country to the north - a power that could be the legendary Holy Grail. The Rajthanans have already assembled an army to capture Mahajan. Jack has with nothing to lose now. He agrees to lead his own men, disguised as porters for the conquerors, on the same gruelling march.

Their hope is to find a weapon that will free England from her oppressors. But they will find something even more powerful.