There are different risks here. Risk of losing money on the stock market, risk of the purchasing power being eroded by inflation. You need to differentiate these in your thinking.
Premium bonds will preserve your capital, but do almost nothing to mitigate against the erosion of inflation. So your 20k will still be there, but will be able to buy even less than now. The same applies to any vehicle which essentially holds your money as cash e.g. interest bearing accounts including cash ISA. This is useful if you need the money soon and you wouldn't want a sudden drop in the markets (like the 30% drop in March 2020) to really affect your purchasing power, with no time to recover. Inflation erosion isn't a big risk over one or two years.
Invest in the stock market (and there are more or less risky ways to do this), and your money may well keep pace with or even outstrip inflation and grow. This is important over longer time frames, because the risk of inflation erosion is a bigger one over more time). And the risk of a drop is less of a problem over longer time frames because you have time to recover (many broad based market tracking investments recover within a year).
But what to invest in? NOT individual stocks (risky as not diverse). Either ETFs or funds, that have low fees, and match your risk profile and timeframe. The inherent diversification reduces your risk. Whatever you choose here, think of it as your sweets.
Then you have your sweet wrappers, or the product in which you keep your investments, e.g. pension, LISA, ISA, JISA. These have different ts and cs, as has been indicated above. Some have free "sweets", like the LISA, as mentioned above. FWIW I'd be LISA and ISA for the 19 yo, dripfeeding across the LISA to the 4000 limit each year. Who would turn down 25% more free money? For the others, similarly, into a JISA up to the limit, and again next year. Then at 18, drip feed into Lisa's.
All of this indicates there is more to this decision than you will get from this Mumsnet thread. However, probably the biggest opportunity here is the educational one. Take your 15 year old and 19 year old along for the ride. The 19yo will have to be opening and managing for himself so will have to be alongside you. Help them learn about compounding etc. These are lessons that will last the rest of their lives long after the money is spent. There are blogs and podcasts (e.g. meaningful money). Get them to choose their company & fees (e.g. vanguard, hargreaves lansdown, ajbell), their funds. Take the time to get it right. Educate yourself in the process. It is completely worth it.