I've looked at this again. Barmaid is right - the money will be a legacy from your grandmother's estate. I assumed from your question that the money was to be put into a Trust, but on re-reading it I am not sure that this is what you intend. There is no reason for it to be in a Trust unless you & your father wish to do that.
As regards Tax Credits, the capital is the amount of the legacy - i.e. its value at the date of death. When you receive it you should also receive any interest which has been earnt on the capital since her death (i.e. while the estate was being wound up). The executors are responsible for paying any income tax due on that interest and will only pass the net (after tax) amount to you. The reference you are quoting from the tax credit booklet is - I think - simply saying that you must treat as your investment income the net amount of interest which the executors pass to you plus any tax the executors will have paid on it (because it is the gross income that is used for tax credits, not the net).
Other than that, you treat as income all the gross (before tax) interest the capital earns after you have received it.
I have looked at the HMRC Tax Credits Manual about investment income. So far as I can see (& it is not at all clear) any income you receive from a trust is treated in exactly the same way as any other investment income.