Are you refer to there 2 year contract ? I believe it not free it more like Rent to Own but I haven't ask them about this.
In a way Voom doesn't really count as it was all about HD only channel which many people didn't care for there channel selection at time where Orby TV on other hand it is gear more towards what many people want and who don't care for sport, music radio, ppv, ondemand and count other worst less channel.
But as you said there no guarantee that the rate will remain low after all it no diff then any online Media and TV Steaming service which all ready start bump up the price how ever there a few that will romain free.