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It's not necessary to submit a detailed itinerary unless the consulate explicitly asks you for it, but you will need to list the activities you intend to undertake while in the UK.
In particular, the application form asks (among other things):
You should list briefly the activities you will be doing (e.g. visiting family, shopping, which tourist sites you want to visit, etc.), and disclose which family member(s) you will stay with, or which hotel(s) you plan to stay in.
Before you complete the visa application, you should review the application form guidance to find out how to answer the questions, as well as the supporting documents guide to find out what documents you should submit, and what documents you should not submit. For instance, it instructs you to not submit hotel and flight bookings.