Bank Rate stands in for the three-month yield; the Bank of England's daily database has no short-tenor par yield.
The ten-year nominal par gilt yield minus Bank Rate. A ten-year yield below the policy rate for a quarter means markets expect cuts, which has preceded UK recessions.
Threshold source: Yield-curve rule with Bank Rate as the short end, since the Bank of England publishes no daily 3-month par yield. How thresholds are chosen
Using the policy rate as the short end makes the spread jump on decision days rather than drift with expectations.