As I said legal maximum for an unlicensed air rifle is 12 ftlb which from changing round the above formula, gives the approximate values as follows:-
A .22 pellet weighing 14.4 grains, maximum permissible speed is 612 ft/sec
A .177 pellet weighing 7.9 grains, maximum permissible speed is 826 ft/sec
The corresponding figures for a pistol are 433 ft/sec for a .22 and 584 ft/sec for a .177
The pellet weights used in the above calculation are typical weights for the sizes of pellet but you must always check the actual weight of your pellet before performing your own calculation