I think unless you invest at least 25k+ in premium bonds, your highly unlikely to get the same kind of returns you would in, say an ISA at a guarenteed 5%
If you’ve already maxed out your ISA allowance for the year then putting money into premium bonds would start to make sense, if you are looking at paying tax on savings interest if the money would otherwise be held in a regular savings account.
From the MSE calculator based on a 50k investment:
Premium bonds at 4.4%
Return of £1,950 (with average luck)
Savings – This shows how much you’d get if you put your money in savings or ISAs:
Top savings at 5.05%
Return of £2,220 – Odds you’ll earn less with Premium Bond 70.4%
Top cash ISA at 5.17%
Return of £2,585 -Odds you’ll earn less with Premium Bonds 85.8%
*remember though, with less than 50k in premium bonds, the chances of winning get smaller, say with only £1000 invested, over 1 year, with ‘average luck’, your return will be a big fat zero!
https://www.moneysavingexpert.com/savings/premium-bonds-calculator/