CV19 will be different to Spanish Flu, but they are the only pandemic we have had in modern times. Spanish Flu was in Europe from around March 1918 to June 1919, although there were still some deaths in early 1920, so there was some sort of herd immunity after 15 months of the virus. There was no vaccine. It was believed around 30% of the population caught the flu with around 0.5% of the UK population died from it (228,000). Like CV19, Spanish Flu was deadly because partly because it spread so quickly and it quickly attacked the lungs, in some cases an infected Spanish flu person was dead within 24 hours of catching the virus, turning their lungs into a rice crisper.
Like BR14 I believe over 10% of the UK population has been infected, most without fully realising it. We know the virus was in the UK in early to mid February so it probably infected a lot of people before lock down officially started on March 24th.