The CAA medical section has a list of various drugs and brand names which are approved for aviator consumption and, consequently, a list of those that are not. I would suspect that some Beta blockers are permitted while, because of certain side effects, quite possibly connected with blood pressure, others are not.
Reference to Aeromedical /LGW or to one's own AME is by far the best course of action.
It might perhaps be an idea to invent a condition such as a prostate problem, which might require treatment with Beta blockers rather than having to admit to such a degree of stress. I rather fancy that the CAA would be unimpressed with an aviator whose worry factor was so high that he had to take medication for that condition whilst continuing to fly or to hold a Class 1.