Looking at that list on the Apple support site, it appears that only the iMac G5 might be 120V only. Whatever the model, examine the label (or equivalent stamped molding) on the rear of the unit. It should tell you the supply voltage range the unit will accept. If it says something like "90 to 250V" or "100 to 240V," then it's one of the auto-switching supplies and will work directly from a British 240V outlet.
I'm not a Mac user, so I'm not familiar with these units, but is the power connected via an IEC C13 cord, the same as the regular PC desktop? If so, and it will run on 240V directly, then you can just swap the power cord for a British one, like this:
http://www.maplin.co.uk/module.aspx?ModuleNo=1176&doy=12m7These cords are available widely in High Street electronics stores. You could also just use a plug converter with the American cord, but to be honest many of the cheap plug converters are of extremely dubious quality, and it would be better to just use a replacement U.K. cord.
If the iMac is a 100-127V-only type, then you will need to run it via a step-down transformer (which are also often referred to as converters, just to confuse the issue).
The main problem is with the cheap "transformerless" converters. They appear to offer good value for money in terms of how much you pay for a given wattage, but the penalty is that they don't provide exactly the same type of power as you get from the wall outlet.
For computers and other electronic devices, you should stick with a simple transformer.
http://www.airlinktransformers.com/http://www.newmarket-transformers.co.uk/
