If the U.K. thought it should send both the Prime Minister and the Speaker to represent it at the G7 the other states would object that the U.K. was over represented. If the mighty USA sent both the President and the Majority leader of the Senate that too would have seemed silly or unfair. Yet if you look at the photos of the meeting of the G7 you see nine people present. Sure enough the EU had sent not one but two Presidents, the President of the Council and the President of the Commission.
This absurdity famously spilled over into a row between these same two Presidents in recent negotiations between the EU and Turkey. The President of the Council who I think is technically the senior bagged the one large official chair for the EU rep, only for there to be very public complaints from the President of the Commission. The EU needs to get its act together and decide in each meeting or negotiation which of its 5 Presidents leads the delegation and sits in the one chair they should be awarded like the countries present.
There is at the G7 a more serious and worrying issue. The EU has five of the nine seats, sending three member state Heads and two Presidents. On an issue like vaccines which is a crucial issue at this summit the EU has run the policy for its member states, so on that issue just one EU rep should engage with four independent countries. If these intimate gatherings of a few leaders of powerful countries are to be valuable they should not be slowed down by the EU giving five versions of what they will do on vaccines.
On issues where member states have some powers but have to work under the legal and policy framework of the EU as in green matters and economic policy it would be helpful if they settled in advance who was in the lead and who could speak for them. There will inevitably be much groupthink and common policy between all 5 so it does not need all 5 present and talking to represent that one viewpoint. Decisions of the G7are usually by consensus, not vote, but having a majority of the voices could distort the debate and give the EU view an unjustified numerical advantage over the US or U.K. view.