The best thing to do here is obtain an oven thermostat and place it in the oven to check the temperatures.
It's possible that the built in thermostat has gone faulty and is switching off way too early.
If you can confirm that the thermostat is cutting out too early, eg. cutting out at 125 instead of 200 then it would indicate the most likely cause is the thermostat on the appliance.
If it needs changing, you would have to isolate the power to the appliance. Take it out of the kitchen unit and take the top and rear panels off.
Inside the oven you'll find the phial for the thermostat clipped to the top rear, release this from the clips and push it out of the hole it comes through.
Take off the knobs and buttons, release the facia (may be clips, may be screws) and then take out the screws holding the thermostat in place (if it's a combined switch and thermoststat then the thermostat can be unclipped from the switch without taking the facia off).
Fit the new thermostat, run the capillary tube and phial into place. Clip the phial back inside the oven. Reassemble, replace in it's built in position and try it out.
Hope this helps.
If you need further help, just post here.
Thank you for the very fast payment. It is appreciated.
When you've done the tests, let me know by posting on this thread. You don't click on Accept again as you've already paid.
I can then confirm if the diagnosis still stands or whether we need to do some other checks.
Good luck with the repair.