Both arguments are valid.
Why should Rishi Sunak pay a lower marginal tax rate on his £2m income (never mind what his Mrs rakes in) than I do on my much more modest income?
No, raising CGT for investors in UK companies to income tax levels is dumb. Even Starmer is talking about creating and attracting wealth now, although he doubtless will put up taxes once PM, probably both income and capital.
As to your 350 multi-million and billionaires, there is risk they will move their domicile and wealth (possibly major companies and employment) out of the country. It's a general strategy that Dennis Healy proved does not work. On a similar principle, Ireland owes much of its current prosperity to lowering corporation taxes for multi national companies to base there.
Sunak paid a lower rate because his salary came from capital gains and not, for example, employment or investment income. Maybe a good government tax advisor could try to catch more of this, but raising all CGT to income tax levels would ultimately risk bankrupting the country because lack of investment would cause companies to leave the UK and go to a more tax and investor friendly jurisdictions, mostly US