more to the point why do it. From what you have said there appears to be no reason to fit one certainly not to go to all the bother of fitting a hull anode and wiring it up. Then there's the issue of the material. Both zinc and aluminium will rendered ineffective if the boat is moored in fresh water while magnesium is too reactive to be used in salt water. Cardiff bay though is presumably brackish and therefore Ok with aluminium.

If you have a prop made of a suitable alloy it should not suffer any corrosion. Pitting is a bit odd though as normally dezincification is the problem if any.

I think this is a case where the rule "If it ain't broke don't fix it" should apply and why drill two holes through the hull for no good reason. You say your surveyor has pointed out that there is no anode. That's a fair comment I suppose since there is not one but if he recommends that you fit one then you are rather caught between a rock and a hard place!

You can always keep the situation under review and if there are any signs of the prop going pink, indicating dezincification, it would be the time to fit an anode. Difficult to see when its painted blue though!

I assume you are on a mooring and not in a marina with shore-power connected permanently otherwise we will be into discussions about galvanic isolators!