Why would fishing boats from Europe want to sail all the way to the coastline of Canada across the Atlantic? You're 100% right in that we don't have to accept their demands, that just means that we don't have to trade with them which will be painful for us. Despite what Farage, Rees-Mogg and Johnson told you back in 2016, they don't need us more than we need them. Most of the countries don't really care for trade with the UK, the likes of Croatia and Hungary have very little trade with the UK. The problem is, when two countries/organisations negotiate, the country/organisation with more power and things to offer can make higher demands. It's why the EU has been successful in trade deals with Japan, USA, China etc. South America are starting to create such a trading block as well. The UK alone has very little negotiating power in comparison to USA and the EU. It's why we got a poor trade deal with Japan recently, a worse trade deal than what we had while we were within the EU.