I can answer that one - since I’ve just been through same situation in practice.
Assuming they are separate schemes - you can draw on them independently. You can take up to 25% tax free from either scheme. Main thing with the DB scheme is that taking the tax free allowance will trigger the pension to start - so you will then only be able to put in 4K per annum still into your DC.
You can take the tax free amount 25% - from DC but the remaining will move to a draw down account - check with your provider - but you do not need to drawn down until you want to - at which point the 4K limit applies.
as with any financial advise - best to speak to a financial advisor.
short answer to your question is - yes - draw on your DC (100k) so £20K tax free and use your DB (300K) all for a pension. If they are different schemes
In my case I pumped all I could into my D.C. scheme using backdated allowances to avoid excess tax on input - I’ve then taken 25% tax free straight out again - from my DC scheme. Then next tax year will take the tax free amount from my DB schema which will trigger the DB pension to start next year (or I may defer another year if we don’t need the cash straight away).
I want to minimise my income by living on capital release from my investments and pension plans + DB pension income.