To follow-up on my points above in case anyone else is doing the same: “Account Number” is your Degiro username and “Account Title” is your name (i.e., name of the Degiro account holder).
@Strabor is correct. You then send an email to Degiro at customers@degiro.ch and they provide an “Outbound Transfer” form which you then complete and send back to transfers@degiro.co.uk along with the Interactive Brokers document that’s generated once you complete the process described above.
So while this all sounds fairly straightforward, I then received a fairly worrying email from Interactive Brokers three weeks (!) after submitting my documents to Degiro:
Unfortunately Degiro is not cooperating. Despite having a transfer order from you they were requiring additional information and documents from us which we have provided. But now they are not responding anymore despite several reminders. Unfortunately there is nothing more we can do since Degiro slowed down the process and made it difficult by requiring more and more documents and not responding now. Please contact them and advise to reply to us. It would probably help and they would cooperate if you could put some pressure on them.
So in case anyone is on the fence regarding whether or not to use Degiro, I would strongly suggest that you don’t.