The OP is asking "can" an insurance company say no, not that they have.If the sole reason they are refusing the claim is because you hold a prov lic then I suggest you contact insurance federation or ombudsman.
I've yet to see anything convince me either way. The convention is that they pay, I presume they might/will pay in this case, but could they refuse? If failure to notify insurance company of penalty points can cause refusal (previous AAM threat) I'd see a chance insurance companies may use prov. licence (possibly under the "licence to drive" as mentioned above, a prov. isn't a licence to drive on the motorway) as a potential reason too. I'm not saying they will, I'm not even saying they can, but I certainly wouldn't put it past them trying it.