It would be hard to say 100% if they are still connected to the central heating without looking at it. But usually those pipes should be blanked off.
If the pipes have open ends then you can be pretty certain that they are not connected to anything.
There might still be a gas pipe in there which will need blanking off at a convenient point.

Hope this helps in any way

If the pipes are still connected to your central heating system they will get hot when your radiators are on. Also, the gas pipe is usually smaller than the flow and return of the central heating pipes, and normally nearer the ground.

Any gas pipes in the area of the chimney breast should be checked and tested by a gas safe engineer and the fire place should be sweeped maybe even re lined

You need to get somone to check, if the old flow and returns are in the chamber I would get them removed. Normally on BBU installs the out side of the breast is boarded to hide the pipework so I'd terminate these pipes to this point. Also a lot of fireplace chambers where BBUs have been installed are left open on the inside so this will be blocking up 1. These are structural factors
2. To stop any CO entering the room or avoids.

The old stainless steel liner will need to be removed and I'd recommend you install a pot if 1 is not on top the stack and a gas cowl to protect the flue from wind, rain and pests.

Good luck with the project