It is not that people are less concerned about brands in Spain.
The situation is more complicated than that for Xiaomi.
People in spain have lower salaries than in the UK, and while groceries and rent are cheaper, electronics are more expensive.
Xiaomi has a very dedicated fan base in Spain, and was a big seller way before Xiaomi came to Spain "officially". One of the top sellers, with forums dedicated to the scene, etc. most just came direct from China, with some low quality resellers.
Now they are selling in Spain big time, and the reasons are clear:
-Better band for the buck. WAY better.
-Top of the line phones are trully top: ceramic phones, noch free full fron screen,etc
-Middle range phones are great, with great battery and quite cheap
-Cheap range is quite good.
-they all have the appearance of great support. They support phones 1/2 years more than Samsung, add cheaper price, long (albeit slow) support and nice phones.. no wonder they sell well.