Both my 'on-the-fly' spell checker and the one in my Opera browser allow ME to add words to their list, so I don't get 'beeped' at when I decide to type thru instead of through.

As to UK spelling vs. American...we in the colonies are fairly thrifty, and so many of our spellings are shorter....(leaving out that 'u' must save...oh...tons (not tonnes)..of printing ink and megabytes of HD space every year... ☺