My grandfather served in 2/10 Baluch Regiment, under British Army during the 2nd World War.
What was the fate of those who were loyal to the British?
Any record details with British Army HQ?
I emphasise that I know nothing about researching the Indian Army and its successors but this might be a start - more definitive responses welcome.
Reading Wikipedia, it seems to me that in WW2 the 10th Baloch Regiment would have been part of the (then) Indian Army. On that basis, according to the FIBIS Wiki, the detailed personnel records will be with the National Archives of India - some comments about research with them are on that page.
So far as I can see, there will be nothing in the UK for your grandfather's personnel records. My understanding is that any Indian Army records held by the UK in the India Office collection at the British Library will only refer to "European" nationals.
The 2nd Battalion may be referred to in War Diaries etc. of the British Army but those references will normally be to the unit as a whole and not to individual soldiers. That's the normal situation for any War Diaries.
The UK's National Archives Catalogue (known as "Discovery") reveals that the UK does have various papers about the Baloch Regiment (NB - I used the spelling "Baluch" to do this search). These are more like regimental histories and they are not personnel records. For instance, one document is at the National Army Museum Templer Study Centre, for instance, and is described thus: "At the close of World War II it was decided that accounts should be written of the parts played by the Indian troops in the conflict, and this collection is chiefly related to the projected volume on the Baluch Regiment. The work in fact never appeared, largely for political reasons connected with Pakistan's Independence."
The only records in the TNA Discovery Catalogue that I have found relating to individuals, are in relation to recommendations for awards - for instance, Lieutenant Abrar Hussain of the 2/10 Baluch Regiment was recommended for an MBE in 1947.
The Baloch Regiment is now part of the Pakistan Army - to what extent historical records were transferred from India to Pakistan, I don't know. If you contact the National Archives of India, that should probably be the first question to ask.
NB - I have tried to use the spelling "Baloch" as per Wikipedia - this is apparently a post-1991 spelling, while "Baluch" was an earlier one.