https://google.com/ncr (ncr == no country redirect). It'll also set a cookie that'll prevent the redirects in the future.
Google stopped respecting 'ncr' quite a long time ago. If it does still work for you it's a lucky coincidence. I've used ncr previously but it stopped working.
Except that somehow it doesn't work reliably.
This problem hits me every time, I know about /ncr, so I visit it, and then it works for a while. Until it doesn't. It's beyond frustrating.