I've been trying the baking soda + white vinegar home oven cleaning procedure and it seems to be working for me - almost the same results as a professional service.
If you want someone else to do it for you, we've previously used the help of Magic Pro Cleaning in London.
Their phone number is: 017 5338 6166
Their work is amazing, always left our cooker spotless every time. Actually one of the cleaning technicians advised us that we can handle the job on our own using the baking soda and vinegar method. Amazing people.
Hope I've been helpful!
Have a great day!