Removing gas fire surround

I'm in the process of replacing my fireplace with something a bit mor
modern. Having taken the old gas fire out (which was done by the ga board incidentally), I'm wondering if I can remove the old chimne breast, remove the firebox surround and then close off the cavity flue
Having exposed the firebox, it is solid concrete with TX11 stamped o the back. It appears to be two pieces, a top and bottom. Neither of m neighbours has this, but neither were living next door when theirs wa removed. So I don't know if this was a major or minor exercise. I looks like it is a drop-in unit, but I'm concerned whether it's chase back into the wall.
A work colleague has a website and has kindly uploaded a picture fo me, here's the picture Has anyone see this type of firebox before? Are these normally recessed back into th wall?, is there some way I can tell?
Any help appreciated.
p.s. for the observant, the trailing wire shown in the picture i disconnected
-- Grahame
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