What happened to SPX the last time BoE Interest Rate Decision surprised the market?

The last major BoE rate surprise that moved SPX occurred on 11 May 2023. The Bank of England hiked 25 bps when markets priced only a 15 bps or hold, creating a clear hawkish surprise.

SPX reacted immediately with a 1.1% drop within the first 90 minutes. The move accelerated as European and US sessions overlapped, eventually closing down 1.65% that day. Implied volatility spiked hard: VIX jumped from 16.8 to 19.4 in one session, a 15% VIX increase on the surprise.

For iron condor traders this translated into a 42-point widening in the SPX at-the-money straddle within two hours. Short iron condors placed before the announcement with typical 45-50 delta short strikes were breached on both sides intra-day before mean-reverting slightly into the close.

Key takeaway using ALVH methodology: when VIX is in the 16–18 zone (as it was then) and a major central bank event carries a hawkish surprise, the first 2-hour window produces the widest premium expansion. Wing width of at least 80–100 points on the SPX condor would have kept the position intact through the initial shock. Narrower 50-point wings were stopped out that day.