Cadburys is very good, but now owned by Kraft, although the recipes haven't changed.
Terry's is also nice and is UK too (although the company was also bought by Kraft (now Mondelēz International))
Out of the American brands I suppose the best one is Mars. You can't fault Maltesers, Galaxy, Minstrels, Mars Bar or Snickers.
Then there's Nestlé who do well for the Swiss. I think Nestlé is my choice for white chocolate.