No such thing as an aerobatic endorsement in JAAland yet, although it might be coming. But even then, you'd be flying PiC legally, so would still Log PiC, even if you might subsequently get in trouble for flying aeros without an endorsement.
In our man's Cub - you need differences training for tailwheel signed off in your logbook. After that, it's perfectly legit to fly his aircraft and log PiC, so long as he doesn't.
If your friend is an instructor, you can log PuI whilst he can log PiC until you have this endorsement, but since it's a permit aircraft, only if your friend wasn't paid to instruct you, which presumably he wasn't. (Strangenesses of UK PtF regulations!).
G