In William Butterfield's original design, the mosaics included a large panel above and around the arch to the chancel, which depicted Christ in Glory, flanked by St Peter and St Andrew. It was, however, never completed due to lack of funds.
Later, in both the 1930s and 1990s, investigations were undertaken to determine the feasibility of completing Butterfield's design, but, in neither instance did funds allow; hence, today, this wall is simply painted white.
The drawing here is what the architect envisaged.