Is it possible to use it for insulation somewhere, say in the attic?

If shreded - it could be used for stuffing toys and cushions. You can offer it to a toy store, or a furniture restoration shop - they will use it for seat cushions etc.

We used old foam one year to make an awesome Halloween costume. We sculpted it into the shape of a Ninja Turtle, used a hot glue gun to put it together, and spray painted it green. It looked really good!

make it new like other sofa

how can you a old bags

If the sofa is older than 1988, it most probably would have been constructed with Polyurethane Foam (PUF) which would make it a fire hazard.

Furniture sold after 1988 should contain flame retardant chemicals that make the foam safer to reuse.

We took apart our beloved sofa this summer as well. I think I’ve used almost ever part of the foam and fluffy stuff. I used the fluff as stuffing for toys and pillows. The foam I used to make cushions for a built-in sofa that my husband and I made. I basically took the foam and cut it to the sizes I needed, wrapped them in old plastic bags (for waterproofing, we have children) and made removable covers. They turned out really nice!

I used my parents old bit of sofa foam as a liner for my recurve bow case. Cut to shape it allows all my bow bits to rest neatly inside without falling about and getting damaged

make a dog or cat bed from a large sofa cushion by hollowing out the middle a little and make a cover to go over out of sheepskin

Not sure if you still have the foam but have you tried going to freecycle.org.uk. Basically people put things on there they want or (like in your case) things they don’t want. Then if someone wants it they will contact you and they will come pick it up