Well, first of all, on a British forum you will of course get a self-selecting group who are comfortable using English. Most people have functional levels of English, so they can understand regular conversational English just fine, watch movies, read news and such. Lots of people aren’t comfortable with speaking, because we don’t do that nearly as much as read, write and listen, and we also tend to find our own accents incredibly dorky. Younger people tend to be much better at it than older generations.
As to why, there are lots of reasons.
Mandatory English lessons in school from ages 9-15, and most study a bit of English in “gymnasieskolan” (high school). We take a 3rd language as well, but English is the big important one (I took German for 4 years and had a good grade, but I barely remember anything).
Lots and lots of American and English movies and television. Only children’s films and TV-programs are dubbed, everything else is subtitled.
Music. There’s very little international popular music that isn’t in English here. A lot of the music produced here has English lyrics as well (in part because English sounds “cooler” to us, and in part because we’re a big music exporter).
For my generation and younger: Lots of gaming. Games generally aren’t localised at all (It’s started happening more lately. I don’t know why, I’ve never heard anyone ask for it).
The Internet, and technology in general. I barely ever visit Swedish sites apart from local news. I keep my Windows language set to English, my Android phone is set to English, I get annoyed when Youtube and other sites keep setting my language preference to Swedish. Since so much information and documentation is in English it’s usually easier to just stick to English even if localisation exists.
University literature is often in English.
We’re an export and trade focused country and Britain and North America are big markets for us (and it’s the default for lots of other places), so lots of people have to use English to some extent at work.
We travel a lot. Even within the Scandinavian language countries (Sweden Denmark Norway) there are comprehension issues when speaking, so we often default to English.
Lots of people read books in English. We’re small markets, so often not a priority for translations. If you’re into sci-fi and fantasy you’re more likely to read in English than Swedish. Here’s a picture from my local Sci-Fi bookshop, all those books are in English:
TL;DR: Our contact surface with the English language is huge. Not everyone is great at speaking and writing, but most people are good at comprehension and English is becoming even more important due to technology and culture.