Please note that PRSCustomizer does not work with the UK version of the PRS505.
This is because the UK version has a different version of the firmware.
However, I believe a fix for this is being worked on - so keep checking the
dev forum.
You could use PRSCustomizer now but you would have to replace the UK firmware with the USA firmware. I'm not sure what impact this would have, but I believe for one that dates will then be displayed in USA format MM/DD/YYYY. There may be other unforeseen consequences - e.g., it may impact your ability to buy from the Waterstones store. In other words, you would be braver than me to try it.