The 31st is the deadline for the second payment on account under self-assessment, the half-yearly instalment that catches out anyone who has forgotten it since January. It arrives in the middle of the holiday season, which is precisely why it gets missed, and it applies whether or not the return itself has been filed.
July opens the third calendar quarter of 2027 and is the fourth month of the tax year. It is also the month when annual leave hits its peak: cover thins out, decisions get deferred to September, and anything needing a full team present is quietly moved to the autumn.
The turn from July into August takes the country into the quietest working weeks of the year, when Parliament is in recess, the courts are between terms and a good share of every office is away. July 2027 hands over the start of the holidays to the month that contains most of them.