I understand it might be too much work to implement British English separately, but can we at least have the option for a UK flag, rather than an american one?
For the amount of money I’ve spent on this service, I feel like that’s a minor and fair ask 
Why not the Flag of England instead of the Union Jack?
Absolutely, that works too, and is unambiguously “English” 
Welcome. The flag? I’ve been here for just 1800+ days and have never worried about the flag. Clozemaster is American, it’s the best on the market (at least in my 'umble opinion) and it’s by far the best value for money. If an England flag was chosen there would be endless concerns about it not being British enough so I’m more than happy with my Italian flag and my sort of British flag;-)
Clozemaster is American, but English isn’t… It doesn’t matter to you, but it matters to me, a native english speaker, and british citizen… Not to mention, almost every other service takes it into consideration (You can be sure I notice :D)
Btw I’m as patriotic as the next, it’s just that getting my head around Italian grammar concerns me more than a flag. But at least we’ve shared opinions so that’s all good; -)
We tend to use American English spellings, colloquialisms, text-to-speech, etc., hence the American flag.