As ever when constrained by budget its worth looking at what you loose out on when comparing the affordable reality with the high end "if I win the lottery". Stretching the £s too far to meet a specification at low price often doesn't work out too well, as I know to my cost.
Blows your budget to smithereens by £100 and may well be too large for the SX3 but, if you need a wide opening, the Vertex VJ400 from Rotagrip **LINK** is an excellent vice and the three nut positions give full depth opening right out to 180 mm making it very versatile indeed.
Most vices have essentially square capacity where opening = jaw width.
I'm very glad I saved up for a pair to put on m Bridgeport. Basically a hydraulic vice made affordable by using a screw instead of hydraulics. Same thing is made with hydraulics at over double the price.
I've always had doubts about the Kurt angle lock anti rise system and clones thereof. Suspect most of the Kurt advantages are due to quality manufacture not the geometry. Seems to me that to be properly effective the pull down tongue should be pretty much under the jaw face not some way back. But Chick is the only design whose kinematics I fully approve of.
Realistically if wide opening capability is going to be important you may well be better off getting a smaller vice for vice work and learning to be creative when clamping to the table. I find that for much of my work a smaller vice would have been fine but, working 12" to the foot scale, the wide capability is often used.
Clive