The artillery doctrine is largely useless beyond the first two abilities. And only with anvil to make the base arty useful.
The valentine's obvious problem is that it comes way too late to be useful. In its current state it should be closer to 5 CP (matching the ostheer puma). Even then I'm not really sure what role it would fill. I've never found it that great against inf personally, but that could be because of all the shrecks at that point.
I'd love to say something about the valentine's vet 1, but I've never actually got one to vet 1.
The sexton just has no impact in the vast majority of situations, and certainly not enough to waste resources that could be spent on any other tank that would have an impact. I don't know the stats, but the shell seems to have virtually no damage radius.
The 12 CP artillery overwatch ability should be renamed "turn your map sectors orange" or "I didn't like munitions anyway".
I agree that the CP for the Valentine could get a decrease since it comes out the same time as the Sexton. My thoughts on it is that it can support Infantry Sections in fighting the 222, 251 halftrack, Puma, or Flak halftrack. Though the 80 Fuel seems to deter UKF players from choosing the Valentine due to the preference of the Centaur which is 20 more Fuel, but can deal with those threats and possible anti-tank guns brought from the OKW and the Ostheer.