Whether to avoid collaborating with an academic because it would prevent them from being a reviewer on your post-doctoral research fellow application I'm applying for a post-doctoral research fellowship in the UK, and a key part of the application is finding academic collaborators for the research. I already have two strong international collaborators, but I would also like a UK collaborator.

Now, one potential person -- let's say Prof. X -- would be a great collaboration, but they are also the person who is closest to my line of work in the UK. This means there is a good chance that they would be chosen for peer-review of my proposal, and I am confident they would give me a high score because I know that they think highly of me. Of course, if they are listed as a collaborator, they will not be asked to review.

Given this, I'm wondering whether it is better to take Prof. X as a collaborator to boost the quality of my application, or leave them out because there is a good chance they may be a reviewer and give me a high score. What do you think?
To succeed in a competitive funding application such as this one, you will need to convince not only people who work in your field, but also those working in more distant areas. Therefore, relying on impressing a single professor who knows your work well is not sufficient. As @Roland comments, you are far better off improving the quality of your proposal through collaboration.