There are books with mechs in the US/UK, like Iron Widow, though I'm not sure about magical girls (after a quick search, I've come up with at least one book - Chosen Ones - so those are about too). Either way, as other people have said, these tropes are really popular here these days, with anime and manga being a very standard interest for kids and adults. Personally, I'd recommend trying US agents possibly first, because I think the market for it is a bit stronger there than the UK (with aforementioned books both being US publications).