What’s the best way to deal with this crack?

Hello, I live in a 1890’s house in the U.K. I’m currently decorating my living room, I’ve took the wallpaper off and found several cracks, this is the worst one: https://imgur.com/gallery/pG2llzB

I think the plaster on top may be lime? Would this mean I would have to use lime to repair or could I use something else?

I’ve seen several different opinions online. One being fill in with foam then use filler over that. I was thinking I could use undercoat plaster to fill then skim over. I’ve also seen scrim tape used. I don’t know which is the best method to use?
I’ll most likely be skimming over then painting.
Any help would be greatly appreciated thanks

Ideally chop out all the plaster there as it looks as though it's ready to fall off.

If not fill with filler/plaster, with a fine coat to finish, Toupret is very good.

Blup

Is plaster still adhering to wall nicely? If so you can fill it up with filler. Troupet flexible filler is very good it moves with substrate. However, if plaster sounds hollow when you knock on it you will have to hack the plaster off to bare brick and sort to the brick gap with mortar and patch the plaster with base coat such as bonding or hardwall then troupes or easy fill.

Hello, thanks for your replies. I ended up filling the cracks with a flexible filler and then using scrim tape over that. I then blue gritted and skimmed over. Plaster went on nicely. Ideally like has been mentioned it would have been good to knock back to bare brick but I wanted to avoid that as I’ve already got a million jobs need doing and a newborn at home so trying to one as quick as I can.