If FilmOn go down, you can always try Zattoo. Pretty much the same but with some advantages & disadvantages.
Cons (Let's start from the bad stuff first)
* You will probably need a Smart DNS to work with this, which is very cheap (and gives you access to the BBC iPlayer, 4oD, ITV etc) but not free (approx €3.60 per month)
* Slightly less UK channels (no Dave for instance, but it has all the main ones)
Pros:
* Video quality is actually a bit better for the free service, especially on iOS (iPhone, iPad). Can't speak for Android though, as I have not tested it.
* Video quality under the sub is amazing on iOS (near HD for BBC 1, BBC 2 and ITV 1, and near SD for the rest)
* Sub is a lot cheaper than FilmOn, and it also gives you access to every programme for the last 7 days, plus you can record 250 shows.
There is a full review over at
Eye on-Demand, and really, if you have an iPad or iPhone, get an Apple TV as AirPlay support is the best out there - plus the video quality is much better than on other platforms.
I find the video quality on BBC 1, BBC 2 and ITV 1 via iPad and AirPlay to my may TV, so good, I almost think I have a proper satellite and an HD receiver.