The horizontal and vertical compliance differences presented by the London Decca cartridges necessitate the use of a "stiff" tonearm. Adding weight to the headshell and increasing the counterbalance weight of a tonearm will not compensate for a resonance compliance mismatch and, in fact, can make it worse. Not all stiff tonearms are heavy and not all heavy tonearms are stiff (although most are).
Think of it this way: a PVC pipe is lighter and "less stiff" than an aluminum pipe of equal size, and an aluminum pipe is less stiff than a steel pipe of equal size. A carbon fibre pipe of equal size would likely to be stiffer and lighter than the PVC pipe; especially if the carbon fibres were aligned down the length of the pipe and not wrapped circumferentially.
Adding weight to each end of the PVC pipe won't make it any more stiff. But the added weight, outboard of the fulcrum (pivot point) will almost always increase the resonance amplitude as they swing around the axis.
Think of it this way: a PVC pipe is lighter and "less stiff" than an aluminum pipe of equal size, and an aluminum pipe is less stiff than a steel pipe of equal size. A carbon fibre pipe of equal size would likely to be stiffer and lighter than the PVC pipe; especially if the carbon fibres were aligned down the length of the pipe and not wrapped circumferentially.
Adding weight to each end of the PVC pipe won't make it any more stiff. But the added weight, outboard of the fulcrum (pivot point) will almost always increase the resonance amplitude as they swing around the axis.