...I know myself too well and am liable to forget to tell them in time to leave O2 dead on the day my contract expires; will they accept a letter now even though I have 4 months to run...
Surely someone who has a job travelling a lot in Europe can diarise a reminder for that?
However if you are set on giving notice and accept the likely consequence that on a day you can't remember, travelling in Europe, your O2 phone will stop dead in the water, go right ahead. I haven't studied the O2 terms and conditions but I would be *very* surprised if phoning Customer Services wasn't the only method specified for giving notice. Either they will accept the notice or (more likely) you will be told to phone back later.
Anyway its hardly a smart move. What limited power a customer has over the networks only occurs when out of or nearly out of contract. The smart move is to wait until you have fulfilled the minimum term and *then* threaten to leave, going for the best deal in the marketplace at that time. If you want to retain your number use the PAC process otherwise you will lose your number.