So two parts of the GS(IU)R to consider:
Non- Compliant Terminal position is covered by regulation 27 would be AR, provided always that products of combustion (poc) are not entering the building.
The process for the Assessment of the Risks for poc to re-enter a building are set out in Gas Safe Technical Bulletin TB007.
So, the answer to your point for the terminal you described is AR. However, you would need to be satisfied and to be able to demonstrate that the requirements of TB007 had been taken into account, in making your decision, in the event of any subsequent issues.
Obviously if poc are re-entering the building, then it is ID.