You don’t need a S&S ISA, no. The capital gains allowance is currently £12,300 a year, I believe and I don’t think you need to report a gain under that. If in doubt, always consult a tax professional and never rely on something an idiot like me says, though. I certainly am no expert and this isn’t financial advice.
If you want the features of Plus and think you might at some point benefit from the capital gains protection an ISA offers, Plus is suddenly £7/month, or your ISA is free if you think of it that way.
Personally, I think having an ISA is a good idea. Even if you don’t plan on making a lot. You never know, you might buy into a stock that goes lol. Plus, keep in mind that reducing the capital gains allowance or increasing the rate that’s paid for gains outside of a S&S ISA is an easy way for a government to get some extra revenue after something like a global pandemic.