Highly unlikely you will electrocute yourself by cutting through a lawnmover cable. The mower handles are all isolated and the fuse will trip as soon as the cable is cut. Wouldn't wase your cash.
if there is already an RCD in the CU then there is “no need to waste your cash”, other wise i urge anyone using any equipment outdoors to use an RCD, especially on any device with a spinning blade and a trailing cable –although theres a good change of blowing the fuse, theres also a reasonable chance of only damaging the insulation on the live and you only finding out when you are packing up the cable with you stood outdoors forming a good earth path! Plug in RCD’s are not expensive.