The aforementioned Tetleys used to brew a fine dark mild which in my formative years was always sold alongside the bitter. It became rarer from the ’90s onwards and think it’s gone completely now.
The best pint of mild I ever had was Highgate Dark Mild, made at a now-closed dedicated mild brewery in Walsall. I believe it’s been relaunched at some other brewery in the midlands though I haven’t tried it.