Another Brit. It's all in the context. "Quite" can easily be a strong superlative but it relies on shared understanding between the parties so I wouldn't depend on that interpretation.

It's fine in a spoken context as tone can add sufficient extra meaning. If you can guarantee the listener is expecting an informal register then it probably also comes across as a superlative. "I met him and he's quite unpleasant" would usually imply he was a total c*nt.