You are wrong. In the U.K., which drives on the left and is in Europe, it is not expressly forbidden to undertake (pass on the left) anywhere, including motorways. This is independent from the legal requirement to remain in the left-most lane if possible.

On designated one-way streets, and thus not on motorways, passing on either side is expressly allowed. Of course, reckless undertaking at high relative speed on the motorway may be interpreted as illegal under other more widely encompassing laws.