but you'd struggle to get a job there if you're monolingual.
This is another aspect to learning ESOL. In many countries, if you do not have a qualification in English, you simply cannot get a professional or government job.
In MIL's home country, you need the FCE and IELTS to not only get into university there, but also to work in a typical office environment where you may never actually have to communicate with any English speakers at all. Only being able to speak your mother tongue is extremely limiting, probably on a par to being a school-leaver in Britain without a C or above in GCSE Maths and English.
Put it this way, if you had to have an A level in an MFL to go to university in Britain, you'd probably find that the rate of MFL acquisition in this country among the young would go through the roof. You'd get language schools opening left, right and centre all over the country, and the BBC would probably launch a language channel. 
In MIL's home country, children start learning English as soon as they go to primary school and many are sent to afternoon English language schools throughout their school life. So by 11 years old, most of them have a fluency in English that is pretty comparable to, say, a nine-year-old monolingual British child.
Personally, I think it is unreasonable to expect monolingual British children to learn a second language to an intermediate level when most schools only schedule two one and a half hour long MFL lessons a week at secondary. You need far more regular exposure to a language than that if you wish to acquire even a working basis. When I was learning my third language from scratch, I probably did somewhere in the region of two hours study/exposure every day.
Again, for decades, children of bilingual parents in Britain have been sent to Saturday schools at religious institutions or cultural centres to learn their parents' mother tongue. A number of my British-born friends went to Polish school at the weekend from 5 to 15 throughout the 70s and 80s. British children from monolingual families simply do not have this type of exposure to another language.
I have to admit that this is a subject rather close to my heart because I've taught ESOL in the ME, and had my own struggles with learning my own family's ethnic language (which isn't my second language
).
And when you get down to it, the motivation for learning ESOL for ESOL speakers can be extraordinary. It's very difficult for other languages to compete with the cultural might of American and British music, for example. Whatever we may feel about it, American hip-hop, alt-rock and pop music is very attractive to teenagers, regardless of their global geographical location. And then there's gaming, which is another huge English-language powerhouse.
Monolingual Brits really are at a disadvantage.