Laying £20 at 6.0 carries a £100 liability. The exchange reserves the liability from your balance when the bet is matched, which is why a lay can tie up far more money than its stake.
Liability: what it means in exchange trading
The most a lay bet can lose: lay stake × (odds − 1).
18+. Trading on exchanges carries real risk and many people lose money. Set limits before you start. Support: BeGambleAware, GAMSTOP.