Presumably as above, the AVC is additional to your existing main DB scheme.
You would not be waiving the right to a lump sum from the scheme. You would be waiving the right to receive it tax free. Even if you waive the tax free option, you could take the lump sum and pay tax on it at your marginal rate at drawdown.
You can take a maximum of 25% of a fund as a lump sum. This is first calculation.
Of this 25%, 200k can be taken tax free ( lifetime limit).
The difference between 200k and 500k can be taken and taxed at 20%.
You are in a DB scheme, so you presumably have option to move the main DB scheme transfer value to an ARF. Is this your idea ? Or are you taking a monthly pension and then looking at the avc fund for a lump ? You have many options, which should be outlined in an options statement from the pension administrators.