Hart of Empire by Saul David

The second thrilling installment in the George Hart series is set during the Second Anglo-Afghan War, of 1878-80
Back in England following his heroics in the Zulu Wars, George Hart is summoned to a new adventure when Prime Minister Disraeli asks him to go on a secret mission to Afghanistan, where the British fear Muslim extremists are poised to overthrow the local ruler and threaten the jewel in the Imperial crown, India.
Hart has severe misgivings. Always an outsider in British society, he doesn't like Whitehall's arrogant way of meddling in other people's religious and political affairs, but, desperate for money, he takes the job and descends the Khyber Pass into a strange and violent land. When his warnings are ignored by the pompous British Resident in Kabul, a terrible massacre occurs, and soon Hart is on the run with a beautiful Afghan princess, in a race to prevent an uprising and head off a catastrophic British invasion.