Hi,
I'm an English A. While it's true that language GCSEs aren't so popular any more, I certainly didn't see any shortage of interpreting students on my course: there were probably about 40 of us with English A in my year. And there were plenty of applicants.
I think that people drop out of interpreting in the first year or so after graduating from a Masters course and that's why there is a shortage of English mother tongue interpreters in Brussels. In my (humble) opinion, the Commission is targeting the wrong group in its campaign: it's not a lack of awareness and enthusiasm on the part of potential students but rather a lack of experience and confidence in recently-trained interpreters. I agree with Aymeric that there are tons of young English A interpreters out there, we're just not necessarily very successful (speaking for myself of course).
It seems to me that in the UK the market is more based on Public Service Interpreting and liaison interpreting: from what I have heard it's very hard to get work as a conference interpreter there. Conversely, interpreting degrees in the UK seem to be largely aimed at passing institutional exams (for the EU and the UN), so the emphasis is on C languages. However, the local private market normally requires a B language and PSI skills and therefore graduate interpreters in the UK are not necessarily suitably adapted to work there. Although you have French B if I remember correctly so I guess it won't be so much of a problem for you.
It can be difficult to get enough experience to pass UN and EU tests: I've met a lot of other young interpreters who work for translation companies and do volunteer interpreting for NGOs in their spare time, for example. This can be very discouraging and it's common for people to retrain in other fields. From my Masters course, I think four or five English As are still interpreting: either in Brussels or on private markets in other countries. That's out of about 40 (I think). None of them live in the UK.
I know a handful of English As who actually interpret for a living and lots more like me who are starting out and volunteering and doing other jobs at the same time. Don't get me wrong, volunteering is enormous fun and it's a great way to get in the booth more often, but it can be an expensive hobby
To be perfectly honest, I haven't had a lot of experience so far: I've done tons of volunteer work and dummy boothing but most of my income is from translation. And I regularly catch myself wondering whether I shouldn't just get an ordinary job in a bank (perhaps not a bank at the moment). Last year I said I'd keep freelancing until the end of the year and then reconsider... now I've postponed it til the end of this year... and then we'll see
Anyway, it sounds as though you have a great combination and plenty of motivation, so good luck!