Yesterday I decided that just on principle I would find a fairly robust way to disable this behaviour. I tried various sorts of extensions, e.g. those that block a site opening new tabs, and redirect blockers but none of them had the desired effect. I worked with Google's Gemini AI to come up with a solution, and it seems the developers of the site have made it highly resistant to any methods of overriding this behaviour since everything Gemini suggested failed - until eventually it came up with a script for the (well known and respected) "Tampermonkey" extension that permanantly removes the checkbox, and with it, the extra tab opening behaviour. The script is coded in a fairly generic way which should carry on working if the developers change the site again, or at least be easily adapted.
I barely use national rail but wanted to find a solution just as two fingers to whoever decreed this obnoxious behaviour!