Who taxes an Indian pension
The treaty settles most of this cleanly. Under Article 20 of the India-UK treaty, a private pension or annuity, whether an employer pension, a personal pension, or an EPS or NPS annuity, is taxable only in the country where you are resident. For a UK resident that means the UK, and India should not tax it. So if an Indian payer is deducting tax on your pension, you can usually stop that by claiming the treaty with a tax residency certificate and Form 10F, and reclaim any tax already withheld by filing an Indian return.
The exception is a government-service pension. Under Article 19, a pension for past government service is taxable in India, as the source country, not the UK, though it flips to UK-only if you are both a national of and resident in the UK. So the split is: private pensions to the UK, government-service pensions generally to India. Getting this right avoids both double tax and unclaimed Indian withholding.
How much the UK taxes, and the lump-sum catch
On the UK side, a common belief is out of date. There used to be a rule taxing only 90% of a foreign pension, giving a 10% shelter, but that was abolished from 6 April 2017, so a foreign pension is now taxed on the same basis as a UK one, effectively in full. So do not assume a tenth of your Indian pension escapes UK tax; it does not.
Lump sums are where it gets genuinely tricky. An EPF balance is exempt in India after five years of continuous service, but that Indian exemption does not automatically make it tax-free in the UK. HMRC's recent position, and the way the treaty routes many foreign lump sums, tends to leave an EPF or similar lump sum taxable for a UK resident, and there is no automatic 25% tax-free element as there is for a UK-registered scheme. Because this is unsettled and fact-specific, a lump-sum withdrawal really should be reviewed before you take it, rather than assumed tax-free. A practising CA sets up the treaty claim on the pension, stops or reclaims wrongful Indian withholding, and flags the lump-sum position for careful handling with your UK adviser.