250a is only 2 channel so would be fine. I know UK and USA is different we have higher wattage outlets, but since yours is only 2 channel it's ok from 15A.
I believe Krell changed the fuse internally so limit the amp current so it didn't overheat and catch fire.
If you had a 7 channel, 250W+ amplifier then that's when you need to get 20A, ie with ATI 1802 you're fine, but if you get ATI 6007 then definitely need one.