Is there a dataset of UK battle sites?
At a minimum, Lat/Long, but the more data the better such as who fought who, how many, who won, when was it, was it part of a particular war, etc?
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You can find the locations of registered battlefields in England and battlefield boundaries in Scotland on the data.gov.uk website licensed under the Open Government Licence. Note that these files show battlefields as polygons, so if you want to extract a single lat/lon pair for each then you'll have to calculate the centroid of each polygon.
There is also a Inventory of Historic Battlefields in Wales but I can't immediately see an option to download the data.
Other data about battles might be harder to find because the facts themselves may be disputed. It's not unusual for there to be several conflicting estimates of how many people fought in a battle, for example, while the further back in time you go we may not even know for sure the date (or dates) on which a battle occurred. Even locations might be uncertain – in 2010 it was discovered that the Battle of Bosworth Field happened in a different place to where it had previously been thought to have occurred while the location of the Battle of Brunanburh remains unknown. Your best bet is probably to use the dataset of battlefield locations to generate a list of battlefields in each country and then use that to look up the details of each battle in the list.