I note an error in AB's assertion that the English translations started with their first book in 1969 - we had already had the Beric the Briton incarnation in the Ranger.
This is old, but I've been today to the Astérix in Britain: The Life and Work of René Goscinny
exhibition, running at the Jewish Museum in Camden, London (and closing next week).
Exactly the same assertion was made, even with copies of both Valiant
on display in English.
The wording wasn't consistent between the information panels, but the point that seems to be the important one is that the 1969 translations were "official", and the earlier ones "unofficial".
I'm not sure how true this actually is, as I think there may be more to it than that; both Valiant
are titles from reputable publishers, so the suggestion that they just ripped off the work, and slapped their own translations on, willy-nilly, isn't exactly credible.
However, reading between the lines, I think I may have an idea for what lies behind this.
We know from the information supplied by Amilah here
that the original German translation was problematic for Goscinny and Uderzo, when they discovered that it was being used in a Far Right publication from a neo-Nazi group. This apparently happened because the French publisher was granting foreign rights to the stories through intermediaries, without watching what they were doing.
My guess is that that is what happened in English too - and may have been why the strips were "localized" from their Gaulish setting. In making sure that the German publisher was stopped, G&U may have either shown that the publisher didn't actually have the authority to sell foreign rights, or insisted on having approval for future translations. Hence why the "official", "authorized" translations start in 1969.