The thinnest depth of 4K and 8K TVs that have been available for more than two years is 10mm and the thickest is 100mm. At 78 millimeters, the KD-65X8507 has a quite thick depth compared to most of the other 4K and 8K TVs that have been on the market for around the same time as this TV.
There are a number of alternative 78mm thick televisions currently available that you might want to consider including the Sony KD-65X8509 (£1,049) which has better reviews.
If you don't mind having a thicker depth then you might want to look at the LG 55UC970 (£1,430.09) (72.1mm).
What does "depth" mean?