by flexi do you mean a braided "plumbing" flexi? if so you are all correct, if by flexi you mean a bayonet fitting and hose, they are allowed unless the manufacturer expressely says must be fitted with rigid pipework, however a bayonet hose has a heat limit of about 70 degrees C so if it is behind an electric oven care needs to be taken to ensure the position of the hose isnt subjected to heat above this, however because im saying it is ok in some situations thats not to say i agree, it is laziness of the highest order, and a properly fitted copper supply means the oven fits in no problem, i used to do loads of kitchens as homers and we used to go on a friday night and strip everything out, on checking the MI for the hob and if it was allowed i would rig up the new hob onto a frame and use the cooker hose and bayonet to connect it to leave them the use of the hob overnight, temp work like that i was happy with but never agree it is a finished job,