Now, for my sins, I rate campsites I have visited by Wi-Fi ability, either their own or by mobile signal strength (I use EE and Vodafone) just for the purpose you describe: somewhere to go and work including sending large chunks of data down the line.
Top ones:
CAMC - Ferry meadows, Warwick, Tewkesbury, Wirral, poolsbrook, Knaresborough, Tredegar house, Hereford, Uttoxeter, Malvern, Burrs park
CCC: Oxford, Moffatt, Canterbury, Kingsbury water park, Devizes, Teversal.
Other: White mills Marina (Wellingborough), Abermarlais (llandegog), Cotswold view near Banbury, Eriba glade (near Banbury), Anita's (mollington), Cardiff pontcanna fields,
Just a selection. If I was going to work, and wanted location, depending what quarter of the UK I was in ....
South West Wales and Abermarlais is delightful (over 18), South Wales it has to be Cardiff just for the lovely walk into the city, Canterbury for the South East, North east England I would go to Knaresborough, North west and the Wirral, Travelling over the border and stop at Moffatt. All of those with the proviso that a strong, reliable wi-fi signal is the priority, somewhere to walk and clear the head is important and somewhere nice to eat is also a priority.