Gettysburg has guides "on site" available for hire. They all seem to be very knowledgeable and well worth the time and money.

For other tours, many are advertised in the various Civil War magazines published in the USA. These may not be available in the UK, but can be subscribed to, or perhaps they are available "online".

I don't know what specific area of the country you will be going to, but would recommend the following battlefields: Gettysburg, Antietam, and Shiloh.