Builder Interior Designer ( UK )

After baxi back boiler / Gas fire removal at the lounge attached happen. Need suggestion: (1) Decoration ideas with website links where to buy the items that will not take space eg https://www.youtube.com/watch?v=vsIStXm12sE (2) For bricking up , whether or not it necessary to keep gap for air flow to prevent damp? https://www.youtube.com/watch?v=8R5DCA123mE

Please dont contact if you are not from UK