If you are separating but don't want to start a Divorce then you may consider entering into a Separation Agreement or Deed of Separation.This is a formal document which will confirm, amongst other things, the date of separation, the financial agreement you have both reached and will usually say that it is intended to be the basis for any Court Order made within future Divorce Proceedings.
Once agreed, the Deed is signed by both of you and you will keep copies.
But beware! A Deed of Separation is not legally binding in the same way that a Court Order is within a Divorce. It is fine if both parties remain in agreement but if one of you changes your mind, it might be possible that a Judge within a divorce would overturn it.
To try to make a Deed of Separation as enforceable as possible at a later date you should make sure that both of you have independent legal advice, that there is full and frank exchange of financial information, and that neither party has been coerced into signing the Deed. Aside from this as long as there is no significant change in circumstances (such as the a serious illness on the part of one party) then a Judge may uphold a Deed at a future date.