That's already being done. Unless you have certain genetic markers in your tumor, insurance will deny certain therapies since they are unlikely to work.
The insurance carrier has a conflict of interest. Over in Europe they have public bodies issuing treatment guidelines, but that would be a political impossibility in the US.
As far as I know the UK is in Europe, and my father has recently been denied cancer treatment by multiple providers due to a low probability of success. That's by the NHS and a few private providers. The argument is that he'll have a better quality of life if he doesn't spend the rest of the time he has left going through pointless treatments that won't help anyway, an argument our family didn't initially accept but are certainly coming to terms with.
How is it any different in Europe? They make the exact same decisions.
US insurance companies use treatment guidelines issued by organizations like NCCN (oncology). If they stray too far, they get dinged for not meeting "medical necessity" requirements.
If anything, you're more likely to get an experimental cancer drug in the US than Europe since US insurance companies don't like bad press.