SP stands for Starting Price, it is the price that a horse was trading at just before the off and if you had a bet with a bookmaker and didn't ask for a price then your bet will be settled at Starting Price.
In the old days the SP was calculated by a representative of the Starting Price Regulatory Commission based on the live prices available in the betting ring.
During the Covid 19 restrictions the method was changed to use off course betting as punters were not allowed on coarse or were restricted to just owners and trainers.
The Starting Price Regulatory Commission have now decided that using off course betting to determine the price will be the permanent process.
Of course it is of benefit to off course bookmakers to shorten the starting price on a horse that they have big liabilities about and some are wary of the new process. But even with the old method bookies could manipulate prices by having their man on the racecourse place a large wager on a horse they wanted to have a smaller starting price against.
Let's say, off the top of my head, that the off course bookie had liabilities of 10's of thousands on a horse that was trading at 6/4, they could wager hundreds or low thousands on course and that price would shorten.,
But their liability would shorten much more against the 10's of thousands they risked paying out and so they reduce their liabilities.
Whether the change is good or bad for punters is a subject of much debate, but what can'y be argued is that less money is going to on course bookies because the off course bookies no longer need to send money on course to manage their liabilities.
You can read the full press release on the change to starting prices here
Should you take a price or accept starting price?
If you are new to betting and have new betting accounts then you will probably have a concession called best odds guaranteed, most of the main stream UK bookies offer this (check on a race by race basis because some bookies will exclude some races).
Best odds guaranteed or BOG means that if you take a price at the time you place your bet and the starting price is bigger then they will pay you at the starting price.
This is a great benefit to anyone who bets with bookmakers, but because of that as soon as you show any sign of knowing what you're doing the bookmakers will exclude you from that benefit.
Horse Racing Starting Price Statistics
It used to be that any system, tipping service or strategy had it's results recorded at starting price, but the days when it was possible to make a profit at bookmakers starting price have long gone.
For example if you had bet every horse that raced last month and you bet was settled at starting price then you would have made about 5,000 bets and lost about a fifth of the money you staked.
Betting at SP the odds are stacked against you, because if the market and the bookie are being efficient then you should lose the equivalent of the bookies profit (The over-round)
There is an alternative these days and that is Betfair SP.
Betfair is a peer to peer betting platform that offers price with very little margin, typically 1 or 2%, and they just charge 2% of your profit per event/race.
Where in the above example of 5,000 bets you would have lost over 19% at SP, Betfair SP would see you lose only 2%, which is a much easier margin to overcome with research and opinion.