Data Retention - ISPA calls on ISPs following bombing

According to an article on The Register, UK ISPs have been asked to retain data prior to last Thursday's attack as it is felt that the bombers may have coordinated their efforts electronically (a not unreasonable assumption). Although this request is reasonable it is a bit impractical:

• Does a common format for logs exist? - No
• What volume of data is involved?- an unknown, but the amount of data involved would be huge
• Will any of the data in space controlled by UK ISPs be of any use to investigators?

If an sms was sent from a non-UK phone on roaming in the UK, for example, which network would hold the logs? If you start throwing around scenarios with regard to the movement of traffic it can cause severe headaches. Although I'm sure ISPs will be more than happy to cooperate in any investigations any logs provided could take months to decipher..