Yes, there's no minimum. The cost of raising a child, adopted or not, varies greatly. If you have more coming in than going out then that's a good start, but it depends what you mean by 'to play with'. You've said bills but what about food, car/transport, clothes and anything else you feel you need to maintain your standard of living, whether that's the gym, TV subscriptions, holidays, repairs or a take away latte every day. If you genuinely have £800 spare every month you'd be wise to start saving at least some of it if you don't have savings.
Have you considered how your earnings will be affected after a child is placed? One of you will need to take adoption leave and often social workers will be expecting you to take a year, which, depending on where you work, is unlikely to be a year on full pay. You'll also be asked about plans you could put in place if the needs of the child prevented you from returning to work. If you do return to work and you have a preschool age child, your greatest expense from that £800 will be childcare, even if you factor in funded hours. It depends on the individual provider but nursery bills can really sting and easily run into a few hundred pounds a month, depending on how many days they go.
My advice would be to go ahead with your application, if it's what you want to do, and you'll be able to explore this with your social worker. It does sound like you're in a reasonable position, but there are so many factors at play that you can't put a price on raising a child. Equally I'd urge you to start saving if you're able to as this will be seen very favourably and give you a buffer for what lies ahead.