It would be far cheaper and infinitely more sensible to take the GCSEs and then apply to NATS, if you don't then you face a very expensive time ahead.
Very few, if any non NATS Airports will take you on for your panacea - "employ me and train me", every single one of them will have existing staff who are already in line for training at some time or other, so, playing devils advocate, why should they employ an unknown quantity such as yourself.
In almost every case, trainee ATCOs at non NATS Units have been ATC Assistants for at least a couple of years.
If you can afford the substantial cost and financial risk of self funding, ADI and APS are essential if you want to command a decent salary, APP is highly desirable but not quite as important as it once was. If you get all the ratings you will be a marketable commodity, but there are no guarantees.
Plenty has been said about self funding on these pages previously but the salient points are:
Be aware of the substantial costs involved, (it's not just course costs but subsistance as well and you'll have no salary at all while you are training).
Be aware that you could be chopped at any time if you fail to meet the requisite standards and you won't get your money back, (not good if you've borrowed the money).
You are not guaranteed any employment even if you do get all the ratings.
There's a lot to think about, especially in these economic times but, whichever route you take, if you make it through the training, you'll be doing one of the best jobs in the world.
Good luck.