The UK is one of the largest investors in childcare provision among developed countries. However, the empirical literature shows these investments have not been effective enough in supporting British working mothers; this becomes even more complex in a context where different sources of informal childcare are available. In this scenario, the UK appears as an ideal case study, since it is the country where mothers present the highest preference for informal kinds of childcare among the OECD members (OECD, 2016). We use data from the UK’s Family and Children Study to investigate the determinants of mothers’ preferences for formal childcare and labour supply, whilst also considering the availability of informal childcare. Our results highlight the positive influence, for British mothers, that the use of informal childcare has on their joint decision on labour supply and formal childcare choices. Furthermore, our results show that British married mothers conciliate formal and informal childcare arrangements as complements rather than substitutes.