You really have not provided enough information for someone to provide you with meaningful help. My recollection is that Intro 22 appeared to be an old style IOR quarter tonner design.
In order to shorten the draft of a keel, you need to know the bolt arrangement. Many bolt on keels have keel bolts that end in a hook that is cast into the lead or iron. These typically extend different lengths into the casting, some very close to the bottom. You would need to either make a shallow enough cut to miss the bolts or else drill, and install a new bolt using a pocket for the head.
To maintain a similar stability (for safety) and motion comfort, you generally you need to calculate the weight and the center of gravity of the old keel, and the weight and center of gravity of the new keel and then design a bulb that ends up with a keel with the same vertical center of gravity. This typically means that the bulb is heavier than the lead removed from the keel.
Because of that you typically end up having to beef up your keel bolts and transverse framing.
On this side of the pond, there are companies like Mars metals who can do the engineering and produce a bulb cheaply. I have no idea if there are similar companies in Great Britain.
I will tell you the few times that I have been around projects like this, these type of modifications were not cheap and the boats did not sail as well as it had. In at least one case, it was cheaper to replace the entire keel and sell the old one for scrap than do the mods necessary to add a bulb.
In any event if the mod's are done right the mods are very expensive and yet decrease the value of the boat some. If they are done poorly, they may be a little less expensive but destroy the value of the boat totally.
In the end, my sense is that you may be well ahead of the game, selling your current boat as she is, and finding a shallower draft boat that better suits your needs.
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Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay and part-time purveyor of marine supplies