It is possible to create user dictionaries and make them active as one wishes. If you spellcheck a few times and add these words to the user dictionary, you will be able to make these words not show up as errors.
As Kovid noted, it is possible to fiddle around with dictionaries, as has been noted in other posts on this forum. It isn't particularly easy and it means that it will displace the regular dictionary as you can only have one dictionary associated with a language as I understand it.
I edit a number of British-American works, so I understand your concerns.
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