Piggynose:
To the best of my knowledge, there have never been any truly independent comparisons of the numerous independent colleges that offer distance learning. ICS is well-established. It used to be part of the major American company, Cengage Learning, but it's now owned by a German publishing house (Cornelsen). As far as I'm aware, the standards are high (and, just as important!) they're financially sound.
However many further education colleges offer GCSE English as part of their adult education programmes (usually through evening classes). You'd find them far cheaper, and probably a great deal more interesting, than anything that 'distance learning' can provide.
Chris