SP (Starting Price) is the official odds at which a horse is quoted when a race begins. Bets placed "at SP" are settled at whatever price the horse is returned at the off, rather than the odds available when the bet was placed. This contrasts with fixed-odds betting, where the price is agreed at the time of placing.
In the UK, the official SP is determined by representatives from the major bookmaking companies who assess the prices available from on-course bookmakers immediately before the race. The result is published by the Press Association and used industry-wide. It reflects the true market position of each runner at the moment the starting stalls open.
When SP is advantageous: if a horse drifts in the market — meaning it becomes less popular and its odds lengthen — bettors who took an early fixed price will be disappointed. An SP bettor in this scenario benefits from the longer price. Conversely, if a horse shortens dramatically (perhaps due to stable confidence or heavy ante-post money), SP bettors receive less than the earlier fixed price.
Best Odds Guaranteed (BOG) is a widely offered promotion that removes the risk from this choice. Under BOG, a bookmaker pays out at whichever is higher — the fixed early price you took or the SP. This is one of the most valuable recurring promotions for horse racing bettors and should always be sought out.
Example
You bet on a horse at 6/1 early in the morning. By race time, the horse has attracted heavy support and is returned SP at 7/2. Without BOG, you receive your fixed 6/1. With BOG, you still receive 6/1 as it is better than the SP. Had the horse drifted to 8/1 SP, BOG would pay you 8/1 instead of your 6/1 fixed price.