For further perhaps now required clarification. I didn't say English needs to be eliminated and replaced by Welsh, merely through whatever circumstance, the linguistics policies are reversed and there is a spread of Welsh throughout the Kingdom.
The spread of a language and the absolute domination of a language are vastly different things. Take Yiddish for example, spoken throughout Europe and yet has almost no dominant position.
Purely Welsh becoming the language of power and therefore resulting in it spreading where needed. Much in the same manner of say French was prior to the Tudors or during Tsarist Russia. Spoken throughout but it was not spoken by everyone!