Is it worth it?

At age 50, if you don't have any existing private pension saving then you probably won't be retiring until at least State Pension age (unless due to ill health or redundancy), which for you is likely to be 67.

17 years is enough time to save an amount that will make a big difference in retirement. Using this pension calculator, a person aged 50 earning £20,000 could build up an annual pension of £1,500 from age 67 if they contribute 3% of salary and employer contributes 4%.

Admittedly, £1,500 isn't much, but if all you have is income of £140 p/w then it is an extra 20% income which is a big increase. And a combined employee/employer contribution of 7% is low, so it should be easy to save more than that for many people.

There is a risk that the extra income is reduced due to means-testing, although who knows what the system will be when you reach pension age. However, you can take pensions (based on total pension wealth, not individual pension pots) worth up to £18,000 as a lump sum under Trivial Commutation. Hence you can start saving, and if it turns out that means-testing is a problem, stop saving later and stay below the Trivial Commutation threshold.