Vietnamese herb Cockscomb Mint / Kinh Gioi / Elsholtzia ciliata (or cristata)

This is a fairly obscure question, but has anyone tried to grow Elsholtzia ciliata in the UK? I have managed to root some from a packet of cut herbs bought from an oriental supermarket locally. It is incredibly grumpy and only seems to be more or less happy with as much solar heat as possible, so in a sunny window. There are videos online showing it rooting like crazy and putting on a huge amount of dark green growth in a matter of weeks. Mine has rooted but the foliage is hardly bolting ahead and even after potting, is still a distinct tinge of yellow, as shown in the pics. The puzzle is that there are US websites that say it's invasive in Connecticut. If this is the case, it must surely be frost hardy, so why the sluggish behaviour that I'm seeing. Anyway, if anyone by chance has tried to grow it in the UK, I'd be very pleased to hear from them. Oh, by the way, it tastes lovely, and goes brilliantly with any far-eastern food -- a bit like lemon balm, but nicer.