Most locum doctors are paid either through an umbrella company (you're an employee, paid via PAYE after employer NI and a margin) or through their own Limited Company (only sensible when the contract is outside IR35). The difference in take-home can be 10%+ of your gross, so it's worth modelling both.
Your day rate is billed to the trust; the umbrella deducts employer National Insurance (~15%) and a weekly margin, then pays you through PAYE. You pay Income Tax, employee NI and (if applicable) student loan on the remainder. Simple, but you keep the least.
Your Ltd company invoices the trust. You pay yourself a small salary (often up to the personal allowance) and take the rest as dividends, which are not subject to NI and carry lower tax rates. The company pays Corporation Tax (~25%) on its profit. This usually wins for higher day rates, but adds accounting responsibilities.
If your engagement is inside IR35, a Ltd company offers no tax advantage and you should use the umbrella route. Always confirm your IR35 status with the engager before choosing.