Why don't you chase the cooker hose into the wall behind the cooker ,leave the end sticking out then extend what's there with some speedfit pipe and some gaffa tape . You could use some jointing compound on the threads. Some people use Plumbers Mait but use silicone if you want a good seal. (Joking)
Seriously, nearly all inset hobs are supplied by rigid copper unless stated otherwise by the hob maufacturer.
You will also need to tightness test the installation before and after the work.
If you unscrew the bayonet fitting from the wall this will leave you with a 1/2 inch female thread. At the other (hob) end you will have a 1 /2 inch male or female thread .
You will need : either two of the following:
http://www.speedplumb.co.uk/feed-male-iron-adapters-pi-468.html
OR one of the above plus one of the following:
http://www.speedplumb.co.uk/feed-female-iron-adapters-pi-150.html
or elbow versions of the above ,whatever suits your purpose.
You will then need to make up some pipework in copper to connect the two using soldered fittings in whatever configuration works best.
You must also fit an isolating valve suitable for natural gas on the supply to the hob such as :
http://www.speedplumb.co.uk/mini-bore-valve-p-999.html
You must then tightness test and purge the installation before sliding the oven in.,making sure the back of the oven doesn't foul the pipework-hope you checked this before you sweated those joints!
There are also such things as gas rates to check, and ventilation requirements to be met.
Easy job!
The reason the flexible cooker hose is not allowed is because of the risk of its exposure to extremely high temperatures (in a tightly enclosed space behind a hot cooker) If the hose was ruptured as a result causing a gas escape the consequences could be disastrous.