Some clients prefer native English speakers, but others do not mind, provided you can do the job properly. And there is no need to tell them, unless you think you need to, or if it is relevant in some way when you are providing proofreading
services.
In fact, some clients prefer proofreaders with English as a second language, because they have learned English grammar to a far higher standard than people who experience the rather hopeless English teaching delivery at many UK schools.
We advise non-UK proofreaders to choose their jobs carefully. They should target clients who require high standards of grammar, and avoid those who use a lot of colloquial language.
This previous blog post adds some additional insights.
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