You will need tell them that you need the limit raising, what you’ll need it raising to (include any other transfers you’ll need to make in the day) and who you are transferring to.
They’ll ask you to provide a selfie with you holding a form of ID.
There are some automated responses to check whether your query can be solved without chatting to a member of support. It should be pretty straightforward to get through those questions, there should only be a couple, and speak to a person though.