My idea: Keep your original SD card somewhere safe and get yourself a 4 GB SDHC card (minimum class 4, better class 6).
Get a new bootloader for your One in order to be able to use 4GB SDHC cards. Safest bet is to download Navcore 8.412 official version from Tomtom:
Open this cab file with Winzip or 7zip and extract the file called system (not TTsystem) to your old SD card. Do not use the rest of this cab file as it only works on a Tomtom XL.
Make sure that your Tomtom is properly charged, put the SD card into it and start it. It will update your bootloader to 5.5128 which is widely accepted as one of the better bootloaders.
After this operation, you can shut down the tomtom, take out the old card, store it in a safe place and use the 4 GB or bigger SDHC card.
On that card, place Navcore SE 8.562 and start adding maps. The only limit for the amount of maps is the size of the SDHC card. They will all have their separate folder and you change maps in the menu of your Tomtom.
I would suggest:
UK and ROI 845.2645 or 850.2781
US/Canada 845.2645 or Canada/Mexico 845.2645 or US/Canada/Mex 850.2784 or North America 850.2781
After your trip to America you can replace the NA maps with Western and Central Europe 845.2645 or 850.2800.
The SE Navcore is already patched so you only need to patch the maps.