We lose zero sovereignty if we allow India 500k visas as part of a Trade Agreement because we can decide whether to accept that deal or not...because we're a sovereign nation.
However if, when part of the EU, a deal was stuck stipulating that we must grant 500k visas to India then we have zero sovereignty. The sovereignty would lie with the EU.
sovereignty
/ˈsɒvr(ɪ)nti/
noun
supreme power or authority.
"the sovereignty of Parliament"
If a sovereign power has a policy that excludes further immigration and it makes a deal with India to relax that requirement in a specific way, it has ceded some control over immigration to another sovereign power, in exchange for an equivalent trade-off
To cut to the chase, our deal with the EU required us to cede sovereignty in some areas (but not all) in exchange for tariff free access to the largest trading bloc in the world, and have freedom of movement in 27 EU countries, and have a veto...
It was the best deal anyone had. Without it we quickly return to being the sick man of Europe. The End