Do you think its as simple as someone (who? Prime Minister?) standing up tomorrow and saying 'all xyz dogs are banned - anyone who has gone one, hand them in to (where?) to be destroyed or you're going to prison for owning an illegal breed'...
First, they have to write new legislation to add a breed/type to the banned list.
To do that they have to figure out what measurements fit that breed/type - they'll do as they did with pitbulls and nick the measurements from the AKC or UKC breed standards for American Bulldogs, and then work out what degree of tolerance from that standard/set of measurements works.
That'll probably be down to DEFRA to do that.
Then actually get that bill/amendment scheduled, consulted, read, several times round... before passing it.
Then work out what to do with all the actual dogs - there is an exempt dog register for pitbull types that are considered not dangerous - those have all been to court, proven themselves (expert assessment) to not be dangerous and been added - with pitbull type dogs now that involves the dog being seized and held in kennels until it can be assessed and determined to be 'type' or not, and then dangerous or not.
Hopefully that won't happen and dogs can just be added within a certain time frame IF the owners fulfill certain criteria (being over 18, 'of good standing' ie not having a criminal record or subject to a dog control order already etc).
Those dogs will then have to be on a lead and muzzled in public for the rest of their lives which hampers training, socialisation/habituation (relevant as this increases the chances of accidents/injury/aggression) and can put their lives at risk (you can't take the muzzle off if they're choking for example). They also have to be neutered and insured (only one company offers this insurance I think, it is specialist). If that insurance lapses the dog will be siezed and euthanised.
You're looking at well over a year I'd say before all the 'new type' dogs are either dead or on muzzles/leads... if we lived in a perfect world, which we do not.
In reality, we'll see a huge increase in dumped dogs, Daily Wail stories about people taking their beloved pets to be put down despite having done nothing wrong, and all the irresponsible twats who want a dog to frighten or actually attack people will continue to do as they do already.
We'll also see huge misunderstanding and misinterpretation of the law, from local authority level down to man on the street level, with a huge sector of the population NOT understanding what is or is not legal - and that tends to cause issues too.
Long term if you actually did successfully eradicate all 'xl bully/american bully' types, we would end up with a different range of potentially dangerous, large, heavy or fast and bitey breeds (Malinois, Dutch Herders, Tibetan Mastiffs, Caucasian Ovcharkas, Akitas).
I doubt we will see dog attacks on people go down though, not in the longer term anyway.