UK prices these days start at £1.50 p/m for their light package. https://instantink.hpconnected.com/uk/en/l/v2 - guess you were grandfathered into to their free package, or was a promo with your printer.
Either grandfathered (I signed up in 2019) or it might've been a new change - I left the UK last year so if it was introduced last year, that would make sense. Good to know though, so I don't keep saying "oh yeah it was free".