The National Curriculum documents for England and Wales are available FREE of charge. They are commonly advertised as being around £25 and £10 respectively, but they are both available free of charge if you call the right number. I can't be sure about KS3 and KS4, but the documents for KS1 & 2 are available free.

If you want an English National Curriculum, you need to call the DfES on 0845 602 2260, option 3. Ask for a National Curriculum document and they will send you one.

If you want a Welsh National Curriculum (available in Welsh or English) for schools in Wales, you need to call the Qualifications line (formerly ACCAC) on 0870 242 3207  (for English medium) or 0845 242 3206 (for Welsh medium).

I hope this helps, but I hate to see people paying for a valuable resource which is in fact free.