Mini Roadster engine power reduced

That reduced engine power and check engine light combo sounds nasty! Is the car going into limp mode? A defective throttle valve could definitely cause response delay, impacting fuel efficiency. Have you checked the throttle sensor connections for any obvious damage or corrosion? Could be a simple fix, but a faulty accelerator pedal module seems plausible, given the symptoms you described.

Limp mode sometimes, yeah. Where exactly is this throttle sensor located? And if it is the accelerator pedal module, is that something I could replace myself, or am I just asking for trouble?

Given that you're experiencing limp mode, it does point more strongly towards a throttle valve issue. Replacing the accelerator pedal module yourself is possible, but it depends on your comfort level with car repairs. It's not overly complicated, but it does require some mechanical aptitude and the right tools. Since you mentioned a response delay, a faulty throttle sensor is also a strong possibility. I would advise you to take it to the workshop. They have diagnostic tools to pinpoint the exact cause before you start throwing parts at it.

Update time. Took my Mini Roadster to the shop. They confirmed it was the throttle valve. Replacement cost was 265€. Runs like new now. Thanks for your help!