It costs around £500-£1000 to have your exome sequenced properly, just fyi, with the price falling year-on-year. That produces data that would legit be useful to an NHS doctor. I'm going to take the plunge and have it done privately in a few years once the price drops just a little further, I think.
So the idea is, 23andme doesn't use (more expensive, more accurate) sequencing to fully label all your genes. Instead, it uses (cheaper, less accurate) tests for specific known mutations - called SNPs ("snips") - on specific genes, and the website even throws away a lot of that data when it produces its report for you (though to be fair I think you can download "Raw Data" as part of your package). Anyway, that's totally cool if you want a broad indication of some known risk factors, but it turns out to be a bit too vague to be helpful if you take it to your doctor. At least, that's what the doctor who gave the talk I went to reckoned!
The future is in getting a complete, high-res picture of your gene sequence. It's available now if you're rich and will be actually pretty affordable in a few years. It's a tonne more data, but it's accurate and complete and your doctor can actually help you interpret it.