What happens if I registered with HMRC late?

Register straight away. The penalty for failing to notify HMRC is worked out from the tax you owed and how long it went unpaid, so every week you leave it makes it worse, and coming forward yourself is treated very differently from being found.

If the tax is paid by the normal deadline, there is often no failure-to-notify penalty at all, because the penalty is calculated on the tax outstanding. If tax is owed, expect a percentage of it, reduced substantially for an unprompted disclosure.

You may also need to file returns for earlier years. HMRC will tell you which when you register with a start date in the past.

This is a good moment to get your records in order, because the return you file will need them. If you have been driving without logging anything, bank statements and platform payment summaries are the place to start.