No it's definitely a form of FPTP. It just moves where the post is. In our current system the post us having a plurality of votes in a constituency. Under AV the post would be getting 50%+1 of the vote.
It's not a proportional system as it would still have all the same problems with wasted votes. It would still be plausible for a party like the Greens or UKIP to pick up millions of votes without getting anything close to a corresponding number of MPs.
If party A got 49% of the vote in every single seat under AV, and party B got 51% of the vote in every seat, parliament would be 100% party B MPs and 0% party A. In any kind of "proportional" system this just wouldn't happen.
I did vote for AV in the 2011 referendum as it happens. And I agree that losing the last referendum shouldn't mean anyone has to stop campaigning.
I'm not arguing against the merits of a proportional voting system. I'm saying don't get your hopes up while Starmer is leader. The only things he's made clear in his time as leader is that 1) he's dishonest. His word doesn't mean much. One months pledge is the next months backtrack. And 2) he's not a progressive. On tax, on policing, on human rights. If he's against internal party democracy for paying Labour members why would he be interested in any increased democracy for the UK.