That's not exactly true. The people elected members to Parliament. Those MPs selected a government, with Major at its head. It was a close decision - Major almost lost the confidence of the House - but the treaty was not signed by some rogue autocrat.

Really no different than any other treaty in the UK. Not everything must be put to a public vote. The Executive has the power to sign treaties. Parliament may only advise, or in the extreme, withdraw confidence and call for a new government.