Possibly part of the "extra" shots is that (I heard on a news report) the bottles actually carry more vaccine than the doses they are labeled for. So, there's an extra shot (jab) per bottle, which would otherwise be wasted. I assume they put a little extra in the bottles in case of problems filling the injecting syringe or something? (Seems odd to not ship pre-filled syringes, as they do with some flu vaccines, but...eh. There's probably a good reason. For the Pfizer vaccine that requires the incredibly low temps, it's probably space-wise if nothing else, more feasible to ship it in multi-dose bottles.)
There are a few non-RNA vaccines that are very close to being approved. If you have a mitochondrial or other disorder where you are advised against vaccines normally, and your current option is a vaccine such as the Pfizer or Moderna that has not been tested on persons with your condition, waiting might be prudent. You should be able to get a more traditional vaccine (killed virus, etc.) at some point relatively soon. (Assuming the NHS purchases it and you have their support - otherwise I think paying privately is definitely worth the money at this juncture.) On the other hand, if you can only get one shot of the two-shot RNA series, you do get some immunity. Not as high as if you've gotten both shots, but a goodly amount. Some of the researchers assume, anyway.
https://www.nytimes.com/2020/12/18/opinion/coronavirus-vaccine-doses.html So if you take the shot and have a bad reaction, assuming you come out of the reaction ok you might have significant protection from Covid even without the second shot. Somehow this feels to me like playing Russian Roulette, though.
Since some of the vaccines were designed to specifically mimic the spikes on the virus, it remains to be seen if they will be as effective (or effective at all) on viruses with mutated spikes.
https://www.cdc.gov/coronavirus/2019-ncov/vaccines/different-vaccines/mrna.html It's going to just keep mutating, so the sooner there is herd immunity via whatever means, the less likelihood a truly vicious mutation starts taking people out, as happened in the 1918 Influenza epidemic.
Fingers are crossed all the vaccines work!
Hoping you all stay safe and well.