Alas, HBO want it back now, to rent to other online services.
NowTV has had it for a good long time, but the end date was set when the availability was granted, and to extend it would involve at the least a new financial negotiation between NowTV and HBO.
And if whoever is getting it next has negotiated for exclusive or semi-exclusive UK distribution rights, it may even be that a NowTV extension is contractually prohibited 
