As an owner of a British car with a aggressive cam, I have spent a lot of time looking into Valve seat recession and lead in fuel.
To cut to the bottom line, you may not need an additive at all if you are only using the car on the street.
STP is the cheapest additive on the market, and might help.
But basically, use the car until you notice VSR, THEN get some hardened valve seats fitted.
But as previously stated, with an aluminium head, you most likely have hard valve seats already.