“If somebody goes missing for at least 7 years a court can declare them dead.”
Well, not exactly. They can make various orders on the basis that they are dead – eg payment out of life insurance and unit trust monies. Many professionals are vaguely aware of this. However, it is extremely uncommon in practice, and when it does it is useful to have clear guidance on what to do.
This is provided by the Bayes-Walker case [Bayes-Walker v Bayes-Walker  EWHC 3142 Ch] involving an RAF Sergeant who went missing from his base in Cyprus and was last heard of in Israel. Mr Justice Warren gives a checklist in less than 3 pages of transcript setting out everything you need to know. And so when you are approached you can get it right first time.
Note: this does not apply to the far more common case of the death in an accident when the body cannot be found. If there is clear proof that the person was say on the aeroplane that crashed then a coroner can certify death in the normal way. You don’t have to wait for 7 years.
Nor does it prevent say a divorce occurring on the basis of 5 years separation, or even unreasonable behaviour by not contacting the spouse for some shorter time. Subject of course to the problems of service in both cases.