Add National Grid ESO (#2269)

README.md

@@ -525,6 +525,7 @@ API | Description | Auth | HTTPS | CORS |
 | [Danish data service Energi](https://www.energidataservice.dk/) | Open energy data from Energinet to society | No | Yes | Unknown |
 | [GrünstromIndex](https://www.corrently.de/hintergrund/gruenstromindex/index.html) | Green Power Index for Germany (Grünstromindex/GSI) | No | No | Yes |
 | [La Data Verte](https://ladataverte.fr) | Aggregation of multiple environmental indicators (CO2 emissions, Average temperature, etc) | No | Yes | Unknown |
+| [National Grid ESO](https://data.nationalgrideso.com/) | Open data from Great Britain’s Electricity System Operator | No | Yes | Unknown |
 | [OpenAQ](https://docs.openaq.org/) | Open air quality data | `apiKey` | Yes | Unknown |
 | [PM2.5 Open Data Portal](https://pm25.lass-net.org/#apis) | Open low-cost PM2.5 sensor data | No | Yes | Unknown |
 | [PM25.in](http://www.pm25.in/api_doc) | Air quality of China | `apiKey` | No | Unknown |