As I understand it, the EU27 leadership made the UK a conditional offer with various options to extend the Article 50 negotiation period. Who has to act to accept and enact the extension?

* Can the UK government accept it on behalf of the UK or do they *legally* need an act of parliament first?
* Once the UK selects one option, do the EU27 governments have to formally accept it or has this acceptance been given in advance? Do any of the EU27 governments need parliamentary approval before they can act?
* Can this be done by phone or does it require physical letters which must be delivered and accepted?