Hello. Girlfriend in question here. I wanted to add in my point of view into this thread.
1. I do not want children, and can't quite explain how strongly I feel about this. I'm not maternal in the slightest, and I truly cannot see this ever changing. I have 3 nieces that I love, but that is about it. This being said, it is solid advice to OP that when he/we discuss with a solicitor, we take this into account incase circumstances change in the future.
2. I do not want to get married. Again, something people seem to have missed on this post. I struggle to understand the points of 'careful, she's trying to steal half of your house, so to solve this problem you should get married'. Well - I do not want to steal OP's house, but if I did, getting married and then shortly divorced would be a sure way to get this done, right?
3. I almost had a house deposit saved when I met OP, 3 years ago. (I was around £3k short - still saving at the time). My plan always was to buy my own house, but when OP had his house and we saw a future together, it no longer made sense for me to buy my own, have a mortgage each, and then not be able to afford to move in together / have two houses to sell if we bought one jointly. We made the joint decision for me to move in to OP's home, and form a cohabitation agreement to protect OP's property against any risk - should the relationship not work.
4. My motivations - we see a future together, why wouldn't we jointly pay the mortgage, be debt free in <5 years, and then start saving cash? OP would be mortgage free by 40, myself by 30, and start investing at this point. Most financial advisors would say pay debts first before investing. The difference of 1-2% in a high interest savings account isn't worth being in debt for triple the time you need to be. Yes, overpayment fees will apply, but we'll be saving circa. £50k in interest on the mortgage. The overpayment fees are pennies in comparison. I also want to invest in something myself - I entertained a BTL, but the tax/hassle made me deem it not suitable. I entertained a high interest savings account - but I struggle to justify saving when OP has a mortgage, which we could clear in less than 1/4 of the time, if I started contributing.
5. The point of the original post was not to ask if OP should trust me, it was to ask what else he needs to do to protect himself IF the relationship did not work. As mentioned by OP, we both have a lot of cash at risk and want to ensure that IF anything were to happen with the relationship, neither one of us gets shafted. Everything commenters have mentioned (about risk to OP's house if I start paying) is the exact reason we are going to a solicitor - to make sure this cannot happen, under any circumstances. I'm not overly bothered if I ultimately do not start paying into the mortgage - if OP decides against this, I would understand, and look into other investment opportunities. I just think it makes sense to join together on it and get debt cleared sooner rather than later.
Finally - in case it is not clear - I AGREE THAT OP NEEDS TO PROTECT HIS ASSETS and have heavily encouraged this from the start. I also need to protect my £45k + future payments to a mortgage that ultimately, is not mine.