It looks like Media Force WiFi requires you to connect and then login using a browser.
http://www.mediaforceuk.com/support/
As you've discovered, Chromecast doesn't support 2-step (also called "captive portal") logins like that where you connect and then have to go to a web-page to finish authorization.
If you do a google search for "use Chromecast in hotels" you'll find how people have worked around this using phone hotspots and (my preference) travel routers.
Keep in mind that neither solution is officially supported and most shared networks like that are grossly overloaded so streaming may not be a smooth experience.
Also, you don't really WANT to get connected to a big shared network like that. If you did, your Chromecast would be visible to everyone else on the network and they could cast to it at any time. Using a travel router makes a private network of your own and then nobody else will be able to see your Chromecast or cast to it.
But, before you do ANYTHING, check with Media Force. They may have a workaround and the also may NOT allow you to attach a Travel Router to their network.
