I work in public relations so my entire job is getting the media to write or broadcast about things I want them to (or more precisely, my clients do).
I can't claim to be a total expert, but below is some free professional advice.
1. For a story to be run by the media, it needs to be CREDIBLE, INTERESTING and RELEVANT. You also need TIMING.
2. CREDIBILITY - this is where you're going to have problems. A web forum of people who believe the NHS offers the wrong dietary advice is NOT a credible source. The NHS will be assumed to be competent and scientific documentation validating its methodology. This is a fair assessment. So in the first instance, you need a credible, recognised body with research that challenges the orthodoxy.
3. More on credibility - an email from a concerned person isn't going to cut it. Production teams on papers field hundreds of emails and calls every day while working on extremely tight deadlines. They will also very rarely respond to any of these that don't immediately interest them (and we'll get to that in a moment), even when this contact comes from professional organisations. They almost certainly won't respond to members of the public.
4. INTERESTING - you need to ask yourself, why is this story interesting to anyone but ourselves? Honestly, why should anyone who doesn't have diabetes even remotely care that a few people don't think the NHS provides the right dietary advice? Sorry, but that's the tough question you need to ask before even thinking of going to the media. Diabetes only affects 2.5m people in Britain and is regarded as largely a self-inflicted condition. You'll need to get over those two massive hurdles.
5. Facts. You're going to need them. Can you prove the NHS diet is wrong? Can you extrapolate the cost to the NHS of not changing the dietary advice vs. changing it? Can you provide real world case studies of people who've done something different to the NHS diet and had a much better outcome that can be medically verified? Again, you need to have proof. Simply a bunch of people saying "I've lost lots of weight on a non-NHS diet" isn't going to cut it. You need interesting, 'everyman' people who have had bad A1Cs and cholesterol clearly marked and defined on their records while also having a food log, and then a clear distinction of before and after. These people need to be reasonably photogenic (for broadcast), articulate and representative of the general population.
6. You need an expert spokesperson. You need a genuine scientific figure prepared to front the campaign and speak on your behalf. Doesn't need to be a celebrity.
7. RELEVANCE. Why should this story be covered now? I'd suggest doing things near to National Diabetes Day would be a step in the right direction. Or releasing information at the same time the NHS release figures on the cost of diabetes. Producers generally won't see diabetes diet as an issue worth covering when they've 500 other people all pitching other stories, so you need something that they'll be thinking of at the time.
8. TIMING - You need to email and follow up with calls at sensible times. Each TV show has a different time for its planning meeting. Don't call when they're in the middle of producing, you want to get to them about an hour before the planning meeting so your idea is still fresh in their mind.
There's your starter for 10 anyway...good luck...