There are countless means by which to measure social impact, many of which will provide significantly different results from the same data. For a Parish Council giving grants to local charities, the requirements are specified in the legislation, albeit in a somewhat vague and intangible definition.
If you are using Section 137, the expenditure must, in the opinion of the members of the council, be in the interests of, and bring direct benefit to, their area or any part of it or all or some of its inhabitants and the direct benefit accruing to their area or any part of it or to all or some of the inhabitants of their area will be commensurate with the expenditure to be incurred. So the nature of a direct benefit, how many inhabitants should receive it and the meaning of commensurate are undefined, and none of this needs to be based upon tangible measure, as only the opinion of the council is required.
The General Power of Competence, as I understand it, (it is not something with which I am familiar) places fewer limitations on charitable expenditure.