“One of the requirements for naturalisation as a British citizen is to have lived in the UK for five years (or three years if married to a Brit). Time living here in breach of UK immigration law doesn’t count. The Home Office has long taken the view that EU citizens physically present in the UK but who do not have a right of residence under EU law are in breach of UK immigration law. Unknowingly lacking a right to reside is surprisingly common — many people without comprehensive sickness insurance were caught out by this in the past.
Because of Brexit, European residents and their family members have been offered immigration status under UK law (instead of EU law). Getting the new “settled status” does not require proof of a previous right to reside — it is mostly based on simple presence in the UK.”
It just got more difficult for Europeans to become British citizens (Free Movement, link)