So to summarise
You have a 1500W 28L Severin 2036 mini oven.
The oven has a specified voltage of 230V 50Hz.
The oven instructions state that the appliance must be earthed.
The oven is supplied with a power cord that is a 3 core cable with EPR (rubber) insulation and sheathing of a cross sectional diameter of 0.75mm².
The plug on the end is a two pin plug suitable for European sockets (and presumably has a metal plate near the top of the inside face that would be an earth connector) suitable to take a voltage up to maximum 250V and a current up to maximum 13A.
Therefore the oven would take about 6.5A at full load (top and bottom elements heating).
From this the cable is probably just about OK for the fixed load of the oven.(normally classified as 6A but if supplied with the appliance should be OK)
The cable is 3 core flex so it can be safely connected to a BS1363 plug (a normal UK plug). However identification of the cable cores colours and connections would be useful prior to cutting off the plug so that the earth can be correctly identified and so connected to the BS1363 plug.
The plug should have a fuse of preferably 10A (5A or 3A would be too small and 13A might be overdoing it, but would be acceptable according to the general rules on testing appliances).
I would suggest that if you want to use a Euro to UK adaptor that you ensure it is one that is rated for 13A such as
this or
this, as there are many only rated for 3 or 5A or may not continue the earthing.
The best method of approach would be to identify where the cores of the cables go to the existing plug, remove the existing plug and securely and correctly connect a standard UK plug fitted with a 10A fuse.
The oven should then work correctly and safely.