While the (vast?) majority of M&G guns from 1950-1982 in the U.K. are branded as Originals, it’s not uncommon to find them marked as RWS, Diana, or Gecado.

Once Milbro had foundered, ending the British Diana brand, you definitely saw more new guns over here branded as Diana or RWS.

As for the older ones, I suspect a combination of personal imports in their possessions by those coming home having been posted abroad with the Army, colonial service, etc (at times when an air rifle did not fit, in the mind of a customs officer, in the same category as dirty bomb or heroin), or bulk sales on the international gun market between dealers.