Hi,
Your coordinate values don't make sense for UTM in the UK, nor do they look like decimal degrees. They *might* be British National Grid, and if that's what your data frame's coordinate system is set to, that's the default used for xy (text) data. If you instead started with a base map, that would be using WGS 1984 Web Mercator (auxiliary sphere), and those values would end up near the equator around Africa.
Melita