Its obviously safer to pull the fuse or switch the breaker off at the consumer unit, but if you cant for whatever reason, switching off at the cooker unit should be ok.
Its a double pole switch so cuts the neutral as well.
I'd tape the switch over so it cant be accidentally switched on. ALSO CHECK WITH A MULTIMETER THAT THE CABLE ENDS ARE DEAD BEFORE DISCONNECTING.
Edit: Check the Kw rating on your new cooker as well, as some of the new ones are 45A in which case you'd have to replace your wiring, cooker unit switch and fuse/breaker.