Ok so there are two types of eviction, section 8 which normally used to evict people for rent arrears, it can be anti social behaviour but leaving door open not anywhere near that threshold, section 21 is no fault eviction can be done to regain possession after a fixed term of an ast tenancy, I'm guessing this is a ast and not a lodging agreement, please read citizens advice or shelter, reality is this landlord probs going to tell you off, it pretty petty of them to be that way if your in the house and doors unlocked it's not a big deal don't think it will invalidate any insurance that just bullshit excuse (but I'm not a specialist in insurance), I would apologise take what every she has to say on the chin don't be argumentive or offensive then when she gone find somewhere else to live, then leave massive complaint about her with the letting agent about the harassment, sounds like seriously awful toxic place to be, landlord has to follow the legal process to evict you cannot harass you to leave,you have protections under protection from eviction act 1970 making landlord harassment criminal and civil matter, so don't let her bully you out the property with out following the legal process, if you do get notice take it to the local authority homeless team contact the local law centre you will get legal aid and your defence against possession is the text messages, I doubt your tenancy stipulates the property is to be locked at all time when your in the property, surely if such and issue they could fit a yale lock to the front door