While financial claims are available to a spouse or civil partner, they are not available to a cohabitant.
However, you can include an agreement for the periodic payment of a capital sum or any expense during your relationship to ensure that both you and your partner are on the same page about what you agree and plan for in terms of financial obligations during your relationship. A well defined document should include details such as the amount to be paid, when payments end or what happens if one of you has another child.
It is important to note that maintenance agreements may succeed as a contractual term, but cannot be enforced due to public policy objections.