meta tag keywords
I know these are hardly used anymore, but please hear me out...
If i do decide to use them, do they have to be purely words, or can they be phrases?
Example: content="used, cars, sale, london, kawasaki, motorbikes"
content="used cars for sale, london, kawasaki motorbikes"
I'd go with the second option. It's more specific, therefore more targeted. Regardless the fact that meta keywords are not relevant for SEs anymore, I bet you don't want to rank for "used" or "cars" which is very common keyword and says almost nothing about user intent - you can't serve those visitors well without knowing what they actually came for. Most likely, you have nothing for them, as your site seems to be about used cars in London.