In a tournament with blinds at 200/400. UTG wants to bet 600. Is this a valid bet or does bet need to be at least double the BB (800)?
This is not a valid bet. The smallest permitted opening raise is by the size of the big blind, i.e. to 2 BB total, or 800 chips in your case.
Edge case rules:
The only exception is if a player has more than 1 BB but fewer than 2 BB in total (so 401 to 799 chips in your case), in which case he may go 'all in' for whatever he has.
In this latter scenario, subsequent players are still obliged to raise to at least 2 BB total (or go all in, if a lower amount) - they cannot just call the 'under-raise'.
However, the previous rule only applies to pre-flop under-raises below 2 BB. Any other all-in bets or raises (on any street) may be called by players yet to act, even if the all-in player's raise is less than the previous bet or raise.
(These are the standard rules but may occasionally vary in particular games. This does not apply to the main answer at the top, which is a universal rule.)