I have an ArcMap basemap of Britain which has a coordinate system which is National Grid. I am trying to import a spreadsheet of coordinates of sites across Britain which are in decimal degrees (eg 50.933392 -1.5033064). However when I use the convert to x and y tool my data points are all projected into the sea off Cornwall. (Presumably my coordinate system is wrong so it puts the point in metres from a 0, 0 point. How do I sort this out?