score:2
You can apply 3 months before your expected arrival date in the UK, and can typically expect a decision within 15 days (not guaranteed, of course). https://visa-processingtimes.homeoffice.gov.uk/y/vancouver-canada/visits-visas/general-visit-6-months-or-less
Note that UKVI guidance specifically states that evidence of travel bookings is not required to be submitted with an application. https://www.gov.uk/government/publications/visitor-visa-guide-to-supporting-documents/guide-to-supporting-documents-visiting-the-uk UK visit visa validity is almost always 6 months, therefore applicants are not obliged to stick to the date stated in the application and do have leeway to change their travel date after receiving their visa.