My sister has the maximum holding (£50,000). In theory that should produce about 23 wins a year. .
A long time ago we used to have the maximum, and the return was pretty good most years - every month a few envelopes dropped through the door IIRC never more than £100.
We cashed them in when GB's budget sneaked in a stealth tax in an under the line item in one of his budgets. Prior to the change, the Prize fund was based directly on one of the National savings interest rates and tax-free in the hands of the winner After the change the prize fund was based on one of the NS rates less an amount representing the average tax take across income taxpayers. So, while the winnings were still tax-free in the hands of the winner, the winning probability was less. We noticed the difference -the $40k went into PEPS and eventually became ISAs
With £50,000
Winnings Probability
£0 Exactly 1 in 43,251,547,540
At least £25 Virtual certainty
At least £50 Virtual certainty
At least £75 Virtual certainty
At least £100 Virtual certainty
At least £150 Virtual certainty
At least £175 Virtual certainty
At least £200 Virtual certainty
At least £250 Virtual certainty
At least £350 99.3%
At least £400 97.7%
At least £450 93.9%
At least £500 87%
At least £750 24.7%
At least £1,000 5.63%
At least £1,500 1.44%
At least £2,500 1 in 675
At least £5,000 1 in 723
At least £10,000 1 in 1,556
At least £25,000 1 in 3,678
At least £50,000 1 in 8,080
At least £100,000 1 in 17,276
At least £1,000,000 1 in 59,822
https://www.moneysavingexpert.com/savings/premium-bonds-calculator/#result