"I can't accept the argument that just because Russia was provoked means it was justified. And I can't accept the argument from Russia that they had no other option other than to invade Ukraine. Why didn't Putin for example do what he's doing now and reduce energy supplies to the rest of Europe...?"

For the Russians to leverage their gas and oil supplies against Europe at that stage would have made no difference because, despite the fact that it was essentially acting as a NATO catspaw, the Europeans would merely claim Ukraine was an Independent state free to make its own decisions.
The Russians likewise still believed that Ukraine had agency and to some extent that was still true: the negotiations for peace in April were therefore between Ukraine and Russia and not the west.

But of course there was to be "...no show without Punch" and thus on 9 April Johnson made his flying visit to Kyiv to insist Ukraine break off negotiations. According to the Ukrainska Pravda Johnston told Zelensky that the West seriously fancied its chances against Russia:

"Johnson’s position was that the collective West ...now felt Putin was not really as powerful as they had previously imagined, and that here was a chance to ‘press him’"

Offering all kinds of assistance if they continued fighting and no help at all if they reached a peace agreement with Russia, Boris convinced Zelensky and a few days later he announced that the negotiations which had up to this point apparently reached a stage of a tentative agreement were instead at a "dead end".

At that point Zelensky effectively surrendered any remaining Ukrainian agency entirely to the west. It was therefore only at that point would a Russian oil and gas embargo to pressure NATO and Europe have made any sense, but of course that would be entirely unnecessary as Europe's politicians decided to do it for them in a misguided attempt to destroy the Russian economy.