Hi Sue,
Welcome aboard Filipino UK

The simple answer to you question is yes.....the working tax credit can be counted towards income under the case you outlined.

The detail can be found within the Immigration directorate instructions Chapter 8 Family Section FM 1.7A -
Maintenance

Please do take some time to review those rules.
In the meantime here's the relevant extract:-

.....staff considering an application for entry clearance, leave to remain, further leave to remain or indefinite leave to remain must demonstrate calculations detailing the actual financial position of the applicant/sponsor in all cases that fall for refusal using the following formula:

A – B ≥ C

A minus B is greater than or equal to C.

Where:

A is the projected NET income (after deduction of income tax and national insurance contributions);

B is what needs to be spent on accommodation (rent/mortgage and council tax); and

C is the amount of Income Support an equivalent British family of that size can receive.
As it stands right now a couple with no children would need to show £113.70 per week remaining after rent/mortgage and council tax is deducted from net income.