Copies of all documents relating to the divorce should ideally be kept. If this is not practical then you must retain the decree absolute and copies of any court orders, certainly until both parties remarry and preferably for life. It is possible to obtain an official copy of the decree absolute at any time after the divorce (for a small fee), but copies of orders can be harder or impossible to obtain many years after the divorce.
If, years after the divorce, you have mislaid your decree absolute and you can’t recall when it was made or which court it was made in, then you may request a search of the central index of decrees absolute kept at the Principal Registry in London. For a search of any specified period of ten calendar years (or, if no such period
is specified, for the ten most recent years) the fee is currently £40 including, if found, providing a certified copy of the decree absolute.