Hi Joppa,It's the former. You cannot enter UK until the 'valid from' date written on your visa, and that's the travel date you put on application unless it's more than 3 months ahead. If you really want to leave as soon as you get your visa, I suggest putting your travel date no more than 2 weeks from application.
So it doesn't matter if you don't get it exactly right? If you want to travel as soon as put the date as close to your application date as possible? If you put the intended travel date for 2 weeks after your actual application date and they end up taking 4 weeks to process it, that will still be ok?