The reason is that the NHS follows an evidence-based medicine approach. In other words, it only approves meds that have sufficiently convincing evidence that they work (and are worth the money).
There are different types of evidence and they are not equal. This is from the multiple sclerosis research blogspot:
Class1 (the best). Evidence from a systematic review (which includes at least one randomised controlled trial and a summary of all included studies). Examples include those published by the Cochrane Collaboration, the NHS Centre for Reviews and Dissemination and NICE. The evidence from such a review requires careful appraisal as, if well done, the evidence is powerful.
Class 2. Evidence from a well designed randomised controlled trial of appropriate size.
Class 3. Evidence from a well designed intervention study without randomisation. Evidence in this category will only be included if no category 1 or 2 evidence is available. A common research design is the before-and-after study.
Class 4. Evidence from a well designed non-experimental study, e.g. cohort, case-control or cross-sectional studies. (Also include studies using purely qualitative methods.) Evidence in this category will only be included if no category 1, 2 or 3 evidence is available. Economic analyses (cost-effectiveness studies) are also classified as Type IV evidence.
Class 5. Opinions of respected authorities, based on clinical evidence, descriptive studies or reports of expert consensus committees.
LDN probably only has Class 5 evidence in its favour, at best. (The only studies of LDN are small and/or poorly designed, plus they get mixed results anyway.) I don’t know about flaxseed oil in particular, but I believe that systematic reviews of omega oils show no evidence that they work for MS. Same goes for alternative therapies - they have no convincing evidence in their favour.
The problem with the internet, however, is that anyone can post anything anywhere to the extent that it’s possible for some theories or treatments to take on almost mythical status! However, if there is no proper evidence in their favour (and even millions of anecdotal reports would not be proper evidence), they will not even be considered by NICE.
As far as DMDs go, they are variable in how well they slow progression (i.e. there is evidence that the interferons slow progression, but they don’t do it as well as Gilenya and Tysabri), just like they are variable in how well they prevent relapses. In some countries, they are prescribed for CIS because Class 1 & 2 evidence has shown convincingly that they prevent development of MS, just like they are prescribed as soon as people are diagnosed with RRMS regardless of the number of relapses and how severe these have been. They are not prescribed in the UK to this extent because of budgets - pure and simple: we can’t afford them. This is because NICE balances the evidence against the cost-benefits of the drug. If something is highly effective and cheap, it will get approved. If something is less effective, but expensive, it probably won’t. Take Gilenya as an example: good efficacy, but really expensive so NICE reject it. Then NICE do a deal with the manufacturer on cost and it gets approved. One thing though - NICE don’t look for things to approve. They will only consider whatever’s been asked for. So unless a drug company specifically asks NICE to consider approving something (and supplies a strong, evidence-based case for it), it doesn’t happen.
Btw, DMDs do not heal anything: they prevent damage. Not sure who told you they did, but they are wrong.
Generally, once a drug is approved for something, it can be prescribed for anything. This is “off label” prescribing (and isn’t as mad as it seems because all meds have to have detailed safety tests before they get anywhere near approval). This generally requires some medic somewhere to take a punt that X might help Y. If it does, then word gets out and gradually X starts to be prescribed widely for Y, e.g. a lot of anti-depressants aren’t approved as neuropathic painkillers, but they are routinely prescribed for it. If the drug company chooses to, they may apply for approval for that drug to be used in Y. However, they would still have to provide convincing evidence that it works and the fact it is routinely prescribed isn’t good enough. And given that Class 2 clinical trials cost a fortune, it’s usually not worth it!
The moral of the story is: don’t believe everything you read on the internet - most of it wouldn’t stand up to even basic scientific scrutiny.