In the UK you only pay child suport for children that a DNA test confirms are yours. However they will not refund payments you have already made.

After the DNA test

If the test proves the person is the parent of the child, the service managing the case will work out how much their payments should be.

Payments will include:

child maintenance the cost of the DNA test 

If the test proves the person isn’t the parent, the statutory child maintenance service may:

refund the cost of the test (if it was done through the CSA or Child Maintenance Service) refund any child maintenance payments made after parentage was denied 

The statutory services don’t pay back any child maintenance that was paid before the person denied they were the parent of the child.

https://www.gov.uk/child-maintenance/dna-tests

So therfore if in doubt get an ealy test. Or do a single parent test without the mother. Provided you are supposed to be the father of the child I understand you can take a sample from the child. I do not see why not.