This certainly used to be a feature at PlusNet! However, having just spoken to customer support (ie, online chat), apparently it's impossible with the current billing system but may once again be possible when the new billing system gets deployed "Soon(tm)".
Unfortunately I'm leaving on a work secondment at the end of January so I doubt that "Soon(tm)" will be soon enough.
Long-time PlusNet customer here (since 2002) so would really like to keep my account, but also not prepared to pay >£40pcm just to keep it idling along while I'm overseas. Also rather unlikely to return to PlusNet despite being on the whole very satisfied as the lack of native IPv6 is really getting beyond a joke (was part of the trials what, 6 years or so ago now?) so if I lose my current account benefits I'm unlikely to return.
So.. after all that - is there anyone I can talk to to try to get something sorted?