Big downside to allowing these numbers
I'd love to disable all numbers starting 07, 084, 087 and 09 by default since they all carry hefty surcharges (except 0845 and 0870 these days, on BT) - or at least have landline-mobile calls charged the same as mobile-landline, instead of being ripped off for them to pay for "free" handsets.
Charging for blocking them is a scam in itself and shouldn't be allowed - I object to being charged to block anonymous calls, too, particularly when making them is required by law to be free of charge! (And, while typing this, I've just had yet another %&$ telemarketer, number withheld, plugging feed-in-tariffs, completely disregarding my TPS registration on the excuse it's a "government incentive" not "sales".)