I also transferred (LSE ETF: VWRD) a few times before, and I have an IBSG account.
Sharing with you the note I keep for my own reference.
As a good practice, you should always check the information of both brokers to confirm the correctness. I last transferred in Nov 2023 and I got the updated IBKR info by opening a ticket with them. I didn't call SC as was lazy.
There was once I hit a problem, due to a "margin issue" according to IBKR. In fact I have a cash account, not margin. The issue arised because I submitted a transfer when I had a trade that is executed but not yet fully settled. So, I always try to set aside a 2 week of non-trading period if I want to transfer from IBKR to SC. After the last trade in IBKR, wait for 2 business days for the trade to fully settle, then submit the application to both IBKR and SC. Avoid making any new trade in IBKR before the transfer completes.