If he named you as the beneficiary on his life insurance policy then you are the person the policy pays out to - his parents can’t override that, unless they can provide evidence that you are dead, or of a will that expressly recounts his naming of you as a beneficiary. What documents did you have that you gave his parents? Was one of them relating to his life insurance? If so, can you remember who it was with? You can contact the provider and ask who the named beneficiary of the policy is.
Regarding his property, you’re going to require a solicitor to pursue this, particularly if his parents are being obstructive. If he died intestate (without a will) then his daughter will legally inherit the property, held in trust until she is 18, and the right to live there; but obviously if the mortgage cannot be paid, the beneficiary of his life insurance is not you, and the beneficiary of the life insurance does not want to pay off the mortgage, the property would need to be sold, and the proceeds held in trust.
If he had a will and has left his property to his parents, then you can challenge the will on DD’s behalf under the Provision for Family and Dependants Act. This could include you being granted the right to live in the property with DD, although the same constraints around how the house would be paid for apply.
Unfortunately, what your partner told you about what he wanted is meaningless unless he actually named you the beneficiary of his life insurance or included you in his will. I’d also be prepared for the possibility of discovering that he may have lied to you about his decisions and beneficiaries.