Where a couple are not in any hurry to dissolve their marriage but wish to record the financial agreement that they have reached dealing with the financial consequences of their decision to separate, this can be done in a Deed of Separation.
People sometimes refer to this as a "formal separation". There is in fact no requirement to formally record a decision to separate, even for the purposes of the Inland Revenue.
However where a financial agreement has been reached for example the payment of maintenance by one spouse to another or regarding the home, it can give both spouses a degree of protection if that agreement is recorded in a legal document.
Such an agreement would normally be upheld by the Court but could be set aside if there had been a significant change in one party's circumstances, or a change in the law during the intervening period.