You could also both enter into wills to deal with your respective assets if one of you passes away, if it's your intention that your property will go to the other person. You would obviously have to change them again if you split up.
May I ask what your reasons for not wanting to marry are [briefly? I don't want to pry]
Marriage offers protection to parties and security, but also requires the commitment of both, it's like a having your cake and eating it situation...if you are not prepared for both sides of marriage it is not very possible to obtain one.
If you are married he automatically has parental responsibility. If you are not married he will have PR if he is named on the birth certificate or if you enter into a PR agreement with him. He can also get PR by applying for a court order which he would almost certainly get.
No, the advice is get protected by a marriage certificate if you want protection, particularly if you need it.
And it's not only about divorce, you obviously don't wanf relatives you are not much in contact with calling the shots over your partner with regards to medical treatment and care of children if you are ill or die.
People think that a marriage certificate is 'just a piece of paper' - it is far more. If you speak to a solicitor they will advise marriage- mine is responsible for quite a few couples getting married. You don't have to have a big wedding- pop down to the registry office with a couple of witnesses.
NOK is very unlikely to be an issue in UK, where a pragmatic approach will be taken (unless your DP has a difficult and vociferous family). You may however not be in UK when an accident strikes, and not everywhere will recognise a non-marital partner.
Government bereavement benefits are not available to those not married or in CP.
Some private pensions, usually older ones but check, pay nothing to those not married or in CP (but you should get something for DCs for whom DP has PR).
And you should check the ownership of every single major assets, review regularly and keep records of who has paid for what and to maintain what.
You are OK until a tragedy strikes and then you may be in a terrible mess-especially, as meditrina says, it happens abroad. Never assume anything-it is like travel insurance, we should read all the small print but don't until too late, a marriage certificate means that you can relax and it is covered-although still sensible to make a will.