Diagnosing RCD tripping to decide whether to fix or junk basic electric oven
Yes. Get an extension cord (I assume they are all 3-prong in the UK).   Slice down the edge of the cable and remove the outer sheath for about a foot.   Don't cut into any of the insulation on the individual wires (though if you nick ground's insulation, don't worry about it). 

Bind the hot and neutral (brown and blue) wires tightly together with electrical tape.  

Now get a clamp ammeter and clamp the hot/neutral bundle.      The amp reading should be zero.   If that is not zero, that is a ground fault, which explains the RCD trip.   Now clamp the ground wire.  The reading should be zero. If it is not zero and the same number, you have an internal ground fault that has been caught by the ground wire.  

If the non-zero figure on the conductors isn't accounted for by the figure on the ground, you have current leaking in a third direction.  

Either leakage implies a problem with the oven.  Try giving it a good clean, especially in the wiring areas.