You should remove the old back boiler, and drain off and remove any flow and return pipes that possibly lead up to a Cylinder.
There will be a gas pipe somewhere, and that should be removed back to where its branching off another pipe. get a GAS SAFE to do any gas pipe work.
The SS flue liner should be removed and the flue swept.
If you want a plain faced chimney breast then, after doing the above, you could brick up the opening but leave a couple of bricks out for an air vent hole - or frame inside the opening with 4"x2"s (including a couple of noggins).
And then fix one piece of plaster board over the whole exposed area your pic shows, and then tape and skim to match.
Finally, fit a vent over the vent hole.
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