In which country do you trademark?

I sell to the big 5 english speaking countries, but have most of my sales outside the US(so UK, New Zealand, Australia and Canada).

I am thinking about either applying for a trademark in the UK or the US at the moment, thinking that one of them might be enough to start with, does anyone have any experience with a similar situation or any recommendations?

Would a trademark in one country(US or UK) be enough to deter any possible future trademark issues in the other countries?

In terms of a possible future exit, does anyone have any trademark recommendations?