How many ounces is a British "tin" of tomatoes?
It is likely the same as a 14 oz can of tomatoes.

From the Wikipedia article on Tin Cans:


  A standard size tin can is roughly 400g; however, the weight can vary between 385g and 425g, depending on the density of the contents.


UK shops selling tins of tomatoes quote 390-400 grams based on what I'm seeing.

This is equivalent to 14 oz.  American cans will generally say the weight in ounces and in grams if you want to double check.