If you just accept the offer, you end up with German shares held...

If you just accept the offer, you end up with German shares held by Bank of Luxembourg. I don't think they are a brokerage firm, more like an investment bank, so you can't sell them at that point. You could do the form at some point in the future after that, if you don't plan to sell immediately then there probably isn't actually a rush.

After accepting the offer, the form is to transfer the shares into a brokerage account in your name. So Interactive Brokers at that point if you have an account with them. It's basically like a 'wire transfer' of shares as if you were sending money overseas.

I will revisit the form soon and post a full version of it with all my personal details blanked out, hopefully it will help others.