There is something called a "fair dealing" excemption that is recognized for british copyright.
Theres a case that pretty much specifically shows the premier league hasn't a leg to stand on.
Back in 1991 the BBC sued British Satelite broadcasting for showing clips of between 14 seconds to 30 seconds of goals , sometimes in slow motion, and the like from the 1990 world cup taken from BBC footage. The BBC said it had exclusive rights from FIFA therefore the BSB had no rights to rebroadcast the clips.
The court disagreed and found that the short clips where covered under fair dealing provisions and that it was ok to take the small clips and rebroadcast it.
Further under the Restricted Practice Act its possible to show that such exclusivity deals can *sometimes* be considered invalid as anti-trust , particularly if the exclusivity is for pay-to-view events covered under scheduled Listed Events.
In a nutshell, I suspect this is an empty threat and the premier league would have trouble convincing a court that a contract between it and the exclusive broadcaster somehow binds a third party who never agreed to it and thus has recourse to normal rights associated with copyright fair dealing.