I would recommend you to use both lang and hreflang because they will improve your visibility overhaul, as some search engines will target hreflang

https://www.linkedin.com/pulse/html-lang-vs-hreflang-understanding-difference-best-practices-bhor/

And in fact, it's the same for the en vs en-gb choice. Not so different, but some search engine love them better, so use en-gb. It doesn't hurt.