I would not use twin and earth into a plug. And there is no such thing as 30A twin and earth. Cable designed to be installed permanently has a range of current ratings according to fitting method. So only flex which should always be visible and uncovered has a fixed current rating so 1.25mm sq flex is rated at 13A for single phase for example.
Any fixed appliance over 2kW should have a dedicated supply, however there is no reason not to use a plug and socket arrangement so it can be removed for maintenance.
So yes likely I would use flex and a plug and socket but do remember any fixed appliance does not have to have overload protection built in. The manufacturer can stipulate that it is protected with a particular size of MCB or fuse so do read instructions. Also you say oven and often these use mineral insulated elements which can have a problem with earth leakage so plugging into a RCD protected socket could cause the RCD to trip and without a RCD the earthing requirement may preclude the use of a 13A plug.
See:- 5126.96.36.199 Equipment having a protective conductor current exceeding 3.5 mA but not exceeding 10 mA, shall be either permanently connected to the fixed wiring of the installation without the use of a plug and socket-outlet or connected by means of a plug and socket-outlet complying with BS EN 60309-2.
So again do read the instructions you may need a blue plug like used with caravans because of the much longer earth pin they have.
Likely there will be no problem with a 13A plug but without reading the instructions I can't give you an answer and I am sure you can read the hand book as well as I can.
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