The chances of your ISP coming after you is minimal, even if they did they would send you numerous letters.
Honestly, what could they do, let's say you denied all knowledge and said I didn't know these apps were illegal, what can they do? They would have to prosecute every single user of the app, you can't prosecute 1 person for a crime and let off thousands of others that are doing the exact same thing. They will go after the person running the app, it's the same with IPTV, they go after the server owners who are making money.
They have to prove what you were doing and prove you were breaking the law, you would get a stern telling off at worst, the police can't even deal with all the nonces and terrorists in this country, why are they going to care if you have watched the latest film release for free.
People out there are abusing their internet usage for much worse content that you watching a knock off copy of a TV show. It's all scaremongering.
About 4 or 5 years ago I used to go to a market that sold pirate dvd, Wii games etc and it was right next door to police HQ, the market was there for years and people were openly selling stuff. It's the same thing, just online rather than physical items.