To me, investing is always down to risk levels. Yes, ‘just plain interest savings accounts’ such as Monzo’s Savings, Marcus and the like do tend to be low %, but they are also low risk (and you are covered up to £85k if the bank itself fails).
I would then say the next level up would be stuff such as Zopa and FundingCircle which I believe aren’t technically protected, but they do tend to have ‘protection plans’ in place and they try and spread your money across several borrowers so if one defaults, you shouldn’t lose too much.
Once you get into stocks and shares (and even into investment via crowdfunding using Crowdcube or Seedrs) and including ISAs etc - the rewards can can up massively, but so does the risk factor - you could lose everything you have invested. At the top end of the scale, you have things such as being a ‘Lloyds Name’ where the rewards can be really really high, but if a major disaster happens, you can lose everything - not just what you’ve invested, but your home/car etc etc.