Capital letter in brand names.

I have written a story but just have a few questions regarding brand names and when to use capital letters.

For example, I have written the following:

I had some Glenfiddich whisky. (Glenfiddich is a capital 'G' but have left whisky with a lowercase 'w' as it is not part of the brand name.)

I then drank some Tennent's Lager. (Tennent's & Lager are both in uppercase as this is the complete brand name.)

I ate a Mars bar. (Again, from my research the full name is only Mars, so the bar is in lowercase.)

Would you agree with what I've done?