As I explained in your other thread, they should honour your travel date (but it will normally be made valid starting 7 days before the date you put), but you won’t know for sure until you actually get your passport back.
As larrabee said, it is possible to apply for a new 30-day vignette if you can’t travel during the dates you are given, but you have to pay about another £200 and send off an application for a replacement vignette, and it can potentially take another 2 months or so to get it back.
So usually it’s quicker and easier just to travel during the dates you’re given, and then if necessary only stay in the UK long enough to pick up the BRP, then return to your home country until you are ready to move properly.
Honestly, though, there’s not really any point worrying about it until you actually get the visa... because you may well get the dates you requested anyway and none of this will be an issue.
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