What happens if I go over the VAT threshold without registering?

You should register as soon as you realise you've gone over the threshold, and HMRC will backdate your registration to the date you were legally required to register, which means accounting for VAT on sales from that point even if you didn't charge customers for it at the time. HMRC can also charge a late registration penalty, calculated based on how much VAT is owed and how late the registration is. It's best to contact an accountant straight away to work out the numbers and register correctly, as sorting it out promptly generally leads to a better outcome than waiting.