I’ve worked both as employed and self employed. It’s never all gone on tax. You would still make enough if you’re doing regular locum work in addition to your paid employment. It adjusts through HMRC so if you pay more in your paid employment it will work out what you need to pay in your Self employment. You just have to remember to do your own tax returns for the self employed work but HMRC send plenty of reminders.
It won’t all go on tax. You’ll just need to do self employed tax return and you can declare on there your employment income. HMRC always figures it out in the end so you might get overcharged at one point but then you’ll get it back. The annoying thing about self employment is they expect you to pay ahead for next year too, so just make sure you save a lot to pay your tax bill at the end of the year. Unlike employment income everything you get into your account isn’t yours.
It wouldn’t all go on tax, that’s ridiculous, your PAYE income would probably just be taxed at basic rate.