Would you be allowed to examine what happened in the long term? E.g. Henry VIII still had to deal with opposition to his rule, which suggests that Henry VII's monarchical authority was still fairly unstable.
Your point about Warbeck, Lambert Simnel etc is still valid because you could mention how those are clear, concrete examples of Henry VII attempting to solidify his authority earlier on in his reign, and then question just how well he managed that (think about the unrest/dissent in the North etc).