As SS has said, although most sparks frown on this practice the regs do allow it.
I frown on this practice for 2 reasons. The first is your cooker circuit is classed as "fixed equipment" so the circuit may have been designed to disconnect in 5 seconds under fault conditions. As soon as you add a socket the disconnection time must reduce to 0.4 seconds so without knowing more about the circuit it's hard to say if it will comply. In the context of this post it probably will but I would rather you had it checked first. If the cooker point already has a socket built in then it should be OK
The second reason. Although at the moment your sockets are for a fridge & freezer, depending on their location they may need to be RCCD protected and it's unlikely your cooker circuit is currently feed via an RCCD. If you add an RCCD to the cooker circuit you stand a good chance of loosing your frozen food because cookers are notorious for nuisance tripping RCCD's.
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