You charge (and then payover) VAT from your effective date of registration. So if you exceed the registration limit then you have one month to tell HMRC and then start charging VAT from the first of the following month (e.g. exceed during June, tell them during July, start charging from 1st August).
If you registered because you expect to make a full £85K or more in the next 30 days then you start charging as soon as you have that expectation/knowledge.
Of course if it’s to your advantage you could choose to backdate your date of registration in which case you’d charge VAT from whichever date you’ve opted for.