So here would be a chemical supplier who would offer sodium chlorite. Unfortunately, there are no prices, but why don't you just inquire?
www.hichemicals.com/HI_Chemicals_disinfectants.html
Here hydrochloric acid is offered (unfortunately again without a price):
www.mccannchemicals.com/index.php/our-products/inorganic-chemicals
However, since both companies are chemical companies, I think that the prices should be quite favorable.
Hm, that sounds interesting. Among other things, they offer chlorine dioxide tablets...
www.watertreatmentproducts.co.uk/product-category/chlorine-dioxide-water-treatment/
That would also be a source that (allegedly) ships to 180 countries.
uk.vwr.com/store/product/9803901/sodium-chlorite-25-in-aqueous-solution-for-synthesis-sigma-aldrich
And here you still have to mix half of the hydrochloric acid with water and (unfortunately order at least 5 liters), but they also have sodium chlorite on offer (unfortunately also only from 5 liters):
5L Hydrochloric Acid 9% and 5L Sodium Chlorite 25%
www.abbeychemicals.co.uk/products-list-a-z
If all else fails, here would be a supplier that would deliver sodium chlorite from Spain:
www.ebay.co.uk/itm/174443613051?hash=item289da64b7b:g:m3QAAOSwJHxguoBs
And the hydrochloric acid would have to be stretched again
www.ebay.co.uk/itm/313509340635?hash=item48fe9ce5db:g:G18AAOSwcW9ghxOx
But I see that it seems not sooo easy to get the ingredients in the UK. I think this is related to the exit from the EU possibly. But that's just a theory, because that way things that are imported can be better monitored. And after all, no one is supposed to get healthy, and certainly not on a cheap way. But that is again just my opinion. I hope one of the sources or vendors works for you.
