Yes, I agree, absolutely - it is largely a 'how long is a piece of string' calculation!But how do you quantify how much is needed to satisfy your moral motivation? If the law allows and requires you to pay £X per year, how do you know your moral motivation in fact requires you to pay £X+Y per year? And how would you pay it, exactly, when the state does not allow tax 'donations'?
For me personally, at the moment I simply avoid doing anything which functions solely as a method of reducing my tax liability. I attempt to calculate my tax in good faith, without attempting to do anything which feels to me like 'playing the system'.
If my situation were such that I felt my moral obligations exceeded my tax liabilities, I would simply give money to those bodies performing underfunded functions ie; particular charities.