I would trust Tyent ioniser. Two filters get rid of 99.9% of contamination. They are being sold on Amazon, but are the cheapest on eBay.
London has hard water
The zero water intrigues me, but reviews state the filters last about a fortnight in very hard water areas, and there is nothing more convenient than my Brita kettle, shame zero don't offer this
Yes this is true. People even live longer in hard water areas! However personally I would rather supplement for this and avoid the fluoride and chlorine and the Zero Water filtered water tastes fantastic, for tea and coffee and on it's own. Yes the filters are expensive (not quite 2 for 50 I think) but they should last over 2 months each for me and they actually work. I mean if you're filtering your water you may as well get an actually effective filter? I didn't do all the research there may well be more cost effective measures out there. The US of A seems to get many more and better options.
Hard water is actually good for you. Calcium and magnesium ions are really important for teeth, bones and cell-cell signalling. Yes hard water tastes gross, and can scale up appliances, but to talk in terms of contamination is a bit of a misnomer. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3775162/