6.2 KW oven fed by 2 x 2.5 twin and earth

No.

Hi there a 6.2kw oven draws 26.9 amps it is recommended to supply the oven with its own dedicated circuit with an isolation switch

Yes it can be connected, will it work? Yes, for a bit, then the cables will overheat which is dangerous, in the worst case it will cause a fire, so it isn't recommended. Get an Electrician to run a bigger supply cable in from the Consumer Unit.

If the 2.5 x 2 cables are just serving this appliance then it’s just a ring with a known fixed load. Absolutely no problem with that as a design. It’s an equivalent of a ring or could be classed as parallel conductors if they are the same length. It’s unorthodox but I can’t see any reason for it to be a departure from regs ?

Have an electrician determine the design requirements of the circuit and provide a quotation to provide that requirement.

I completely agree with Jon Bateson. There is no danger or departure from the regs, if the two cables are similar in length, terminated at the correct type isolator switch and definitely not serving anything else. But I can also suspect that Coombes is trying to utilize an existing ring mains, so unless you blank off all sockets properly, stay away from this design. Cheers Ivan

In theory yes. But I’d say a big fat No. The problem with running parallel supplies is unless they are exactly the same length, the current will usually flow down the path of least resistance I.e. the shorter cable. Then this can lead to problems like overheating etc.

Absolutely not. Sounds like you're trying to connect it to the ring main. An oven should be supplied from its own dedicated protective device.