Should I sign a statement confirming I have no rights to my boyfriend’s flat?

We live together but I don’t pay rent and I’m worried it will affect our mortgage when we buy a houseQ Shortly after I met my boyfriend in September 2016 he bought a flat. I moved in with him in June 2017. I pay half of the bills, but I don’t pay any rent money, because this flat is his only and I never wanted contribute to his mortgage. When he bought the flat he took out a two-year fixed-rate mortgage which needs to be renewed very soon. The first time he used a broker but th