Betfair Exchange is a peer-to-peer betting exchange licensed by the Great Britain Gambling Commission. Unlike a bookmaker it does not set prices or take the other side of your bet. You are matched against other users, and Betfair charges commission on net winnings in a market rather than building a margin into the odds.
It is the largest exchange of its kind, and for UK users it is usually the deepest available market on British politics, elections and sport. Prices are quoted in decimal odds rather than percentages, so a contract trading at 4.0 is the exchange's way of saying roughly 25%.
UK residents, mainly. Polymarket and Kalshi are not straightforwardly available to UK retail users, and Betfair is the mainstream regulated alternative. Gambling winnings are not taxed for individuals in the UK, which is a real difference from trading the same question on a US venue.
Commission applies to net winnings per market, at a base rate that varies by market and by your rewards tier. Liquidity is excellent on major elections and thin on almost everything else, so a quoted price on a small market may not be tradeable in size.
For where Betfair sits legally alongside the offshore venues, read which prediction markets are legal in the UK.
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