Thanks Gareth!
Yes, I am considering to buy an another Audi to be honest... I had a TT and I'm looking for a 5 doors now.
I went to Audi dealer as you suggested and they provided the service history. They were very attentive and gave me the information I needed. Also, I checked the information in the DVLA website and MOT history.
It was an A4 avant 2016 TDI ultra, but I thought the price was a little high for the condition and mileage of the car. Unfortunately it wasn't this time... but I'm still looking.
What the Audi dealer informed me is that cars after 2016 do not have the service book with stamps. It's an optional item nowadays. All records are recorded online in the Audi database.
Here's a tip for everyone, if you're looking for a car after 2016, go to the Audi dealer and ask for the service history.
Thank you Gareth