It's the old Abbey National (now Santander) sort code. It was not an individual branch sort code, it applied to all their customers, wherever they banked.
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(27*27*27*27) = 27⁴ = 531,441
27 x 27 x 27 = 27³ = 19,683
The last digits of each number are counting down as the first digits are counting up. 09, 18, 27, 36, 45, 54,63, 72, 81, 90, 99, 108
274 = 27 x 27 x 27 x 27 = 531441
7+19