So yeah, england is small, but marketing Wise in Football they are big. And so far, i didn't hear anything making me feel that Bale would still be a star once he leaves. He'll always be popular, but if Madrid thinks they will flood English market with their shirts, they are really deluding themselves. PL fans will find another star to represent their league. If Madrid expect to see Bale's face on every walls in england, again they are deluding themselves.
You're right in that the average English/British football fan couldn't give a toss about Bale. And his homeland; Wales, although they would be proud to have one of their sons playing for one of the worlds biggest clubs, already have their clubs that they follow with a strong passion and loyalty, with clubs like Swansea, Cardiff etc. They are not going to suddenly start supporting Real Madrid just because a Welshman signs for them. Wales is a very small country anyway so even if they were to sell a few RM jersey's, it's not going to amount to much.
I think Beast was focusing more on the global market, particularly the English speaking one. North America is a territory I think they will target with Bale. He won't make much of an impact in S. America as Cristiano, Messi & Neymar have that territory sewn up. Asia could be another market on the radar.
The problem I have with Bale becoming the "New Poster Boy", is with his personality. Although he uses that stupid goal celebration and has even patented it, he is not someone that courts publicity. I suppose that same could be accused of Messi, but he is GOAT, so he's slightly different.
When Beckham first appeared on the scene in the mid-nineties, he was a shy and retiring type until he hooked up with Posh Spice. Then his personality changed, he became more extravert and embraced the celebrity lifestyle and formed himself into a global icon that transcended football. It also helped that he was a good-looking chap.
Unless Bale goes though a similar change, I don't see him having much of an impact, in terms of worldwide fame to non-football fans.