In the UK you need a 110v transformer powered tools because the only corded power tools you can use on site are 110 volt as well (battery tools are all OK). 110 volt power is far less likely to cause a fatal shock than 240v if you have an accident or a cable is caught/cut/gets wet accidentally for any reason. I'm not sure the same rules apply to kitchen fitters though because most of the ones I meet use 240v tools!
The transformer plugs straight into the 240v mains socket and reduces/steps the power down to 110v. There are a couple of factors that will affect which transformer is best for you.
I've always bought 110v power tools because even though I have to carry around a heavy transformer I know I'm not going to get turned away from a job by the site agent for health and safety reasons - because of the power rating of the tools I'm using. I use a twin socket transformer which is plenty powerful enough for the carpentry 110v power tools I use.
110v Transformers are available in lots of shapes and sizes, with varying amp outputs and with one, two or more tool sockets. If you are only going to use the transformer yourself for things like a mitre saw, hammerdrill, planer, jigsaw and other normal 110v power tools then the 3kva 110v Transformer like mine will be more than sufficient for you too. As long as the power rating (amps) on your tools doesn't exceed the one on the transformer you'll be OK.
You can't run a 240v lead from the power socket to your workplace and then plug the transformer in there as the lead still poses a risk to others. So, to go with your 110v transformer you'll want a few 110v accessories. I have an 110v extension lead and a four way junction box too so I'm not plugging/unplugging tools all day. I don't worry about plugging 2,3,4 or more power tools into it because I can only use 1 tool at a time, I won't overload it. If a few people will be working it is better to have two transformers than overload a single one. If you are on a large site though it is likely there will be a large generator for every one to get their 110v power directly from.
Have a question or comment about choosing 110v transformers?