The UK for starters, I have a large choice, from A&A to Zen.

The last mile fibre is provided by a company called OpenReach. I don't deal with them, they sell the service to the ISPs at a regulated wholesale price, it's those ISPs that add their services (like IP connectivity, transit, etc) onto the top

I think that is actually plenty of countries. Most countries do not have an ISP oligopoly like the US, Canada or Australia.

The parts of Australia with NBN (read: about 90% of houses in the country) have choice in ISP. I'm not sure of the split between all the FTTx options, so they may not all count as "high speed".