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"