The question asked though, is "when is my last payment", to which the answer is - 30days after you explicitly give 30days notice or cancel via a PAC.
EE's contracts are not fixed term ones that end on a particular date. They are ongoing contracts with a minimum term - which many people confuse with an arbitrary end date.
Once your minimum term is over, you are free to cancel or reduce to SIM-only with no penalties. But if you do nothing, your contract will continue on exactly the same basis.
