No deposit bonuses are what casinos offer people to sign up. These are really popular with players as you don't actually risk any of your own money. The casino just gives you credit that you play with. The wagering requirements are generally really high, sometimes as high as 100x with roulette not counting at all or maybe just 10%. At that level of play through, you are unlikely to be able to cash out.
These are a nice way to try out a casino though and also to have some fun at the expense of the casino. There is also sometimes a maximum amount you can win too. For example, some casinos only let you cash out £100 max. Some casinos require you to make a deposit before you cash out. That's not too much of a problem because it means you have won something to cash out but it's not as lucrative as just no maximum
Casinos rarely offer no deposit bonuses nowadays. The best you are going to get nowadays is free spins on deposit. For example, Betfred gives 50 free slot spins on a deposit of £10.
New customers only. 18+. Promo code CASINO100. £10 min transfer and stake on Slots at Betfred Casino within 7 days of registration. Minimum 5 game rounds. Game restrictions apply. Max 100 Free Spins. 50 Free Spins on selected titles credited instantly after qualification. Second 50 Free Spins on selected games within 48 hours. £0.10 per spin. Free Spins must be accepted within 3 days. 7-day expiry. Not all titles may be available in iOS app. Payment restrictions apply. SMS verification and/or Proof of I.D and address may be required. Full T&Cs apply.