Add the thickness of your bench to the thickness of the vice base, i.e. bench =25mm thick, vice base = 15mm thick. Bolt needs to be minimum of 40mm PLUS length for washers and nuts, so looking at minimum of 50mm long. Its a small vice so simply take it with you to B&Q. Look in their loose bolts boxes, (or even open a closed one), and select one that will go through the hole and have enough length to go through the worktop and fit a washer and nut on the end. I would suggest putting a washer under the bolt head as well as one underneath so, assuming only 2 fixing holes, you need 2 bolts, 4 washers and 2 nuts. I would use nyloc nuts as ordinary ones have a tendency to work lose through vibration.
Just for information. You cannot screw a bolt into a wooden worktop Bolts have to have either, a nut on the end or a thread cut into the steel if fixing to a metal bench.