Answer: There is an old legal doctrine called “commorientes” which says that where two people die at the same time and it cannot be determined who died first, the younger of the two will be treated as having survived longest. However, you can ensure that the rule does not apply by making a Will that states the other party will not inherit unless they survive you by, for example, 30 days. In any case, if you are not married your partner may not be entitled to anything at all unless you specify it in your Will. If you marry and then die without having made Wills, the commorientes rule is automatically disapplied and the surviving spouse would be the primary beneficiary under rules that apply on intestacy.
Overall it would be best if you made your Wills.
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These answers can only be a very brief commentary on the issues raised and should not be relied on as legal advice. No liability is accepted for such reliance. If you have similar issues, you should obtain advice from a solicitor.