your partner is the legal owner and you have a beneficial interest in the property - to help protect your interests you can put a notice (the caution you mention) on the land registry, this just means that any future buyer will be made aware for your interest in the property. In addition to this you would be well adviced at this point to see your solicitor again and get a simple declaration of how the property will be shared e.g. what proportions of the property "belong" to you as it were (e.g. 75% him, 25% you) this will save any confusion should a dispute that place, it is really important that you do more than just put the notice on as the courts are full of these sort of disputes when couples break up and the outcomes can be unfair to one party (95% of the time unfair to the equitable owner) as the courts are bound to follow case law and cannot simply divide property held as they see fit - in the way they would if you were married.
So please get professional advice on this as it really is very important
undercovers