As your linked info recommends, the 18" size needs to be an XL tyre. The smaller ones don't.
By being stronger, XL tyres can be run at higher pressures increasing the load rating further still. This strength usually comes at the expense of weight and often stiffness. If you don't need the XL type, the standard tyre will probably perform better as the compromises are different, should be naturally better. If you need them, i.e., the manufacturer specs them, the reinforced tyre is required to take the weight or cope with the heating effects of maximum speed, etc. Even if you in the UK should never get near these limits, it is still what should be fitted to remain at or above the design spec.
The unknown is that maybe when the car was designed/handbook was written, there was only an XL model in that size and later a lighter tyre came along. However, without more info, dropping to a standard tyre wouldn't be advisable.