Potoharis(described as "northern punjabis") were readily recruited into the British army because our chieftains had been bought off by handsome royalties. The Chib chieftains of Mirpur are still being paid yearly royalties. Besides that, we were at good terms with British from the very start and had actively helped John Nicholson in Attock and Hazara. Our chieftains were easily bought because they had been rendered fugitive and landless by the Sikhs; their sworn enemies. Baloch and Pashtun chieftains who were independent on the advent of british prevented their tribesmen to join the British army, even though they were labelled "martial" and actively sought.
Besides that, the British were very stringent with the recruitment process. They preferred Baloch, Pathan, Sikhs and "upper caste" Punjabi musalmans. I can talk best for Potoharis, and we were preferred because of our non-sedentary lifestyles, hunting skills(still strong), horse riding skill(northern Punjabis dominate the tent pegging in present day Punjab) and a violent history. Within Potohar, the recruitment was almost completely limited to rajputs, Gakkhars and Malik Awans. The Gujjars, Jatts and Dhund Abbasis(who fell out of favour due to their attempted rebellion in Murree) of Potohar were not recruited into the army at all before world war 2(in world war 2, they relaxed martial race theory even though Winston Churchill opposed the said relaxation).
From central and South Punjab, recruitment was mostly done from Kharrals, Sials and Tiwanas.
@INDIC: I don't take any pride in supporting them, but our forefathers had no choice. Had Sikhs and Dogras not attacked us, I don't think we would have joined them at all as they were not muslim. But the whole anti-brit rhetoric is rich, when it comes from the UP and Bihari people. You people served as cannon fodder for the East Indian company, who later on discarded you, declaring most of you non-martial "rapists". You were their very first a**lickers, who turned on their own countrymen.