Hello, I'm Keith and happy to help you with your question.
I regret to have to tell you that you are barking up the wrong tree. There is no Inheritance tax (IT) rate of 35% in Scotland, nor indeed in the UK. Here is the relevant extract from Scottishwills.com:
'Inheritance tax is a form of tax which is payable on an estate when a person dies. The tax itself, which applies at a rate of 40%, is only payable on amounts above the estate tax threshold. The current estate tax threshold is £325,000. Therefore, if your estate is worth more than this amount, inheritance tax will be payable on the balance at the rate of 40%.'
Always remember that the value of all assets world wide are aggregated and subject to IT. This includes his residence and many people forget this item.
You and the other two beneficiaries can enter into a deed to vary the will and charitable bequests do indeed inflate the 325K level at which IT kicks in, but the surplus over that, will still suffer tax at a flat rate of 40%. Whilst by your variation you have reduced the quantum of taxation the rate will still be estate - charitable bequest - 325K @ 40% IT.
There is no problem in executing an amending deed to vary the original save for the cost of preparation. It is your deed, not the deceased, so within reason you can do what you will and as you suggest it may speed up the process and release funds earlier than having to wait for possible problems with the already selected First Scottish.
So sorry to have to rain on your parade regarding IT rates.