Whats the minimum raise after a straddle

[MARK R]

Blinds are £1 £1 someone staddles to £2 what is the minimum raise. I have always though the min raise would have to be to £4. But thinking about it just now the straddle has only raised by £1 and if you did not straddle but raised UTG then the next person could min raise to £3. Also what is the minimum bets on further rounds.

Mark.