If the police refuse your diversion, you can’t appeal their decision to the courts. You can also ask the police to “review” (take another look at) their decision. You, or your lawyer, should ask either the police prosecutor for your case, or the District Prosecution Manager from the Police Prosecution Service. You don’t have to ask in writing – your request can just be verbal.
You can also ask for a review if you’re offered diversion but you’re not happy about the conditions the police have attached to it.
Note: The rules around diversion aren’t hard and fast, and the police have a wide discretion about whether to offer it. Your lawyer may be able to persuade the police to offer diversion when they hadn’t initially been intending to offer it.