I don't see this as valid S137 expenditure. Section 137 allows councils to incur expenditure that will bring direct benefit to the area and/or its inhabitants and the expenditure must be commensurate with that benefit. The use of the power is well defined. Specifically:-
"A local authority may, subject to the following provisions of this section, incur expenditure on contributions to any of the following funds, that is to say—
(a) the funds of any charitable body in furtherance of its work in the United Kingdom; or
(b) the funds of any body which provides any public service (whether to the public as a whole or to any section of it) in the United Kingdom otherwise than for the purposes of gain; or
(c) any fund which is raised in connection with a particular event directly affecting persons resident in the United Kingdom on behalf of whom a public appeal for contributions has been made by the Lord Mayor of London or the chairman of a principal council or by a committee of which the Lord Mayor of London or the chairman of a principal council is a member or by such a person or body as is referred to in section 83(3)(c) of the Local Government (Scotland) Act 1973."
As has already been stated, your total expenditure under S137 during the current financial year cannot exceed £8.32 muliplied by the number of electors.
Furthermore, your fighting fund of £100k may fall foul of the wider legislation, depending upon its original source.