Currently around 500 deaths/ million. If you need ~60% for "herd immunity", that implies they'd get around 2500/1m, or 0.25% of the population. Pretty gruesome.
Estimated "peak" daily infections was 23 March, with 500k infections. Estimate for daily infections as of 10 May closer to 10k; estimated country-wide Rt at 0.75, still well below 1.
Method seems to be from this 2009 paper, "Bayesian modeling to unmask and predict influenza A/H1N1pdm dynamics in London":
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3215054/
Not equipped to evaluate their method, really; but if it's reasonably accurate, then the UK is not currently really headed towards "natural herd immunity".
Seriously, click the link and look around -- there's a lot of interesting stuff to see.
15-24: 0.0032%
25-44: 0.018%
45-64: 0.28%
65-74: 1.8%
75+: 16%
The last one looks similar to the measured death rates by age group in Germany -- i.e., the COVID-19 deaths of 80+ in Germany divided by people tested positive in Germany was around 18% when I looked before, IIRC.
So nearly the entirety of the "1-3% fatality" comes from the fact that it's so incredibly deadly to the elderly.