Interaction of top slicing and charitable donations

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I have an distant elderly relative who has income (State pension, Occupational pension and savings income) which just exceed the basic rate allowance. She regularly makes charitable donations with my assistance to bring herself back to basic rate status every year.

She has an investment bond taken out in 1999 for £5K now worth £17k and is considering cashing it in (she doesn't need the money but wants to make things easier on her eventual death). I understand top slicing and have done the calculation but don't know how the extended basic rate allowance (because of the charitable donations) is meant to be applied in terms of the extra tax calculation.

The gain is £12000 over 25 years (=£480pa). Provided she makes sufficient charitable donations in 2026/27 to create at least £500 headroom within the extended basic rate allowance, I reckon the additional top-sliced tax on the gain would be £480*20%*25=£2400, rather than the full gain £12000*40%=£4800, meaning that the Top Slicing Relief would be £2,400.

Are my figures correct and does anyone know would HMRC be happy with that approach to the calculation. Is it permissible to use charitable donations to extend the basic rate allowance and then apply the top slicing?

Very grateful for any input.