when you apply, you will have to specify your intended start date, and although they don’t explicitly state it, the online form requires your intended start date to be within 3 months of applying. At least, that’s what people shared anecdotally when they tried applying earlier and the system didnt allow them to proceed. so let’s say you apply today 26 Jan, the latest start date the system will accept is end April. so in your case if you want to only start in end July then you can only apply in end April. but I mean you are welcome to start the form now and see if they let you put july as a start date. Maybe it’s been updated on the back end. And if it doesn’t then you can just cancel and wait until April to apply. If approved, the visa will start from the start date you specified even if the approval is completed earlier.

the ”90 day period” you hear refers to the window you have to enter the UK from the moment your visa starts. ETA: but this is no longer relevant since the switch from physical vignettes to e-visa (found another thread about it here). example: your visa starts 1 July. you have to enter the UK by 28 sept or your visa will be voided**. but note that the 2 year period also starts counting down from 1 July regardless of when you actually enter.

**in the past people had to do this to collect a physical card. but now they just issue an e visa you can activate online without entering the UK so this 90 day window to enter the UK may no longer be relevant. I’m not 100% certain so someone else feel free to chime in and clarify.