St Martin-in-the-Fields is the church on the corner of Trafalgar Square. In Dress to Impress, Jacob climbs its tower to spot the Royal Guards so he can steal a uniform for Abberline.
After parking the Gladstones' stolen carriage, Jacob finds Abberline disguised as a street sweeper. Only royal guards may carry weapons into the Queen's ball, so Abberline needs a guard's uniform. Jacob climbs the church to scout the guards patrolling nearby, knocks one out, strips the uniform and stows the body in a carriage, then brings the uniform back. The memory sits between Double Trouble and Family Politics in the final sequence.
Henry VIII had the first church built in 1542 so that plague funerals would not pass his Palace of Whitehall. Being so close to the Strand, the 1721 rebuild became a favourite of wealthy worshippers in the 18th century.