I have a combination of a HTB cash ISA, Premium Bonds, and S&S ISAs.
The interest on the HTB cash ISA is higher than I would get elsewhere (although still not overly generous), however up to £12k I'll get a 25% bonus if I use it to buy a house, so potentially up to £3k 'free' from the government. I keep this one as it does generate a little interest, I'll get the bonus, and if I'm strapped for cash I can access this instantly. However, you can only put in £200 per calendar month, so I try not to touch it unless I need to.
Premium Bonds are about as safe as you can get, however they're unlikely to do anything spectacular for you. Pros include low risk, easy access to your savings if you need (although they take about 3-5 days to pay you when you withdraw) and you can now invest as little as £25 at a time (it used to be minimum £100). So now I have a £25 DD every week. Con is obviously very little returns in most cases. I've won £25 about 5 times over the last 3 years, with a few thousand held.
My S&S ISAs I've held for short - medium term and include a regular ISA and a LISA. I'm comfortable choosing my own investments and I keep an eye on them. I get the best return on these, but I won't be holding them for too long as I'll be cashing them in next year, all being well.
You can absolutely hold S&Ss in the short term as well, however the pros of increased returns is balanced by the cons of the risk that your capital will shrink if the markets suffer. You can mitigate this risk by diversifying your portfolio geographically and across industries, and keep a good mix of bonds, equities, and alternatives.
S&S portfolios are certainly best when held over the long term, but I wouldn't say they're a bad idea in the short term as long as you're taking lower risks (if you want to save for 6 months time I wouldn't chuck everything I have in a highly volatile global equities portfolio, for example - go for bonds, index trackers etc).