The Prime Minister could, in principle, instruct the Queen (this whole arrangement was abolished before Brian got his mum's old job as we'll see shortly) not to issue the Letters Patent for a new Lord, but Parliament has explicitly laid out the rules for this, so, he is in contempt of Parliament. This seems like an unwise course of action as of course he serves only at their pleasure and even Sir Keir, who has an unusually large majority, has discovered that if they don't like what he proposes they can just ignore him.

None of this matters for the Supreme Court, and thus for about 15 years now. It's true that the Supreme Court's justices are made life peers (its original members were of course already peers having previously constituted the Law Lords, but new members are granted a peerage) - however that's merely a convention, if you don't make them a life peer it makes no difference to their job on the court, it just makes you look petty. I don't even think it's contempt now, because the law saying they should be elevated was repealed - unless the new law also says they must be given a peerage when they get the job, I glanced through it and didn't find that, but it's a huge law because making a Supreme Court was not its main purpose.