Yes it is safe to use the switch. Basically it is there if a hard ground is not provided. The switch attaches a cap to one or the other power leads to the amps ground. It should go to the neutral power side for it to work properly but your ear will hear the best position for it. Think of the switch like on old amps with two prong power cords. The ground switch acts exactly like these old amps if you don't have the third ground position. If you do have the a ground position the switch should be in the middle position which disconnects the cap. Because you are using a step down transformer it may not work as intended as you are isolated from your power but it is safe to try it.
I do have a question though. Doesn't the UK have a ground (you guys may call it earth) connection. If you connect the amps third connection (the odd one) to your earth/ground I think this would solve your noise problem. If you don't know what I am talking about I wouldn't suggest trying this without some educated help. Hope this helps.
