The ninety thousand threshold is not yours
Every guide to UK VAT leads with a turnover figure. If you and your business sit outside the UK, that figure does not apply to you at all.
The rule everyone quotes
A UK business must register for VAT when total taxable turnover for the last twelve months goes over 90,000 pounds, or when it expects to go over 90,000 pounds in the next thirty days. Below that, registration is voluntary.
That is the sentence that gets repeated, and for a UK established business it is correct.
The rule that applies to you instead
There is a second test on the same GOV.UK page, and it has no number in it. You must register regardless of taxable turnover if all of the following are true: you are based outside the UK, your business is based outside the UK, and you supply any goods or services to the UK, or expect to in the next thirty days.
Read it twice. There is no threshold in that test. A first sale of forty pounds to a UK customer triggers the same obligation as a first sale of forty thousand.
Timing, if the threshold route does apply
For businesses that cross the threshold rather than the non established test, registration is due within thirty days of the end of the month you went over. The effective date of registration is the first day of the second month after you crossed.
So going over in the middle of June means registering by the end of July and being registered from 1 August.
Why this one is worth reading twice
The gap between the two rules is where the trouble sits. An owner abroad reads the 90,000 figure, decides they are years away from it, and starts invoicing UK customers from a UK company.
HMRC's own guidance calls this group non established taxable persons and publishes separate notes on it. If that phrase describes you, the threshold was never the question.
The dates for your own company come out of the deadline calculator, and the filing itself is described in the dossier.