I require a list of data on UK cities giving longitude and latitude details. Preferably in CSV or Excel format.
You can probably use OpenStreetMap (OSM) for this. OSM is a open database of geographical features. OSM features are described by tags, that are combination of key-value pairs.
The entry point to look at the geographical features of OSM is this page: https://wiki.openstreetmap.org/wiki/Map_Features, which describes the major features that are mapped in OSM. Look at https://wiki.openstreetmap.org/wiki/Map_Features#Places for the section about places. "Cities" in OSM are be described by
place=city. If you need more little towns, you can search for
place=village, etc. (see the above link for more tags of places in OSM).
To query the data in OSM, you can use http://overpass-turbo.eu/. Use the wizard (one of the top tab), type "city in UK", increase the timeout up to 60s and run the query. Or just click on this share link since I just tried it ;-) : http://overpass-turbo.eu/s/orK. You can then export the result in geojson and convert it to a csv with the city name, latitude, longitude. There is also additional information that you may discard.
I'm not aware of open UK gov data, but it might be an other possible source.
Areas/ boundaries of UK cities and major towns from the Open Geography Portal at the UK Office for National Statistics:
Available as .csv/.kml/.shp
Probably the best source of GB data is the Ordnance Survey - in this case you want the Open-Names product.
Go to GeoNames:
The GeoNames geographical database covers all countries and contains over eleven million placenames that are available for download free of charge.
They have all kinds of support as well.