Can I use Wago connectors to extend a ring main?
Yes, but a few points need to be observed to make sure it complies with BS7671  IET 17th Edition.
Firstly, the terminals must be installed in a Wagobox (when breaking into existing ring wiring). This needs to be UK sourced to ensure you get ones that are stamped with [MF], meaning that it's classed as BS 5733 Maintenance Free box.
To comply with the IET regulations, each Wagobox is limited to 50Amps in total. The compatible terminals, 773(solid) and 222 (lever), are also derated to 20Amps.
Therefore, for a regular 32Amp ring main, you will need to use two Wagoboxes - one at the start and one at the end of the extension. As the load from any socket is shared in both directions of the ring (albeit unevenly), the derated 20Amp terminals are sufficient.
Before:
                         +---------+
     L                   |         |
    +--------------------+  |   |  +-----------+
    |                    | - - - - |           |
    |                    |         |           |
+---+---------+          +---------+           |
|             |                                |
|  Consumer   |                                |
|  Unit       |                                |
|             |                                |
|             |                                |
+---+---------+          +---------+           |
    |                    |         |           |
    +--------------------+  |   |  +-----------+
     L                   | - - - - |
                         |         |
                         +---------+

After:
                         +---------+           +-----------+
     L                   |         |           |           |
    +--------------------+  |   |  +-----------+  Wagobox  +--------+
    |                    | - - - - |           +-----------+        |
    |                    |         |                           +----+----+
+---+---------+          +---------+                           |         |
|             |                                                |  |   |  |
|  Consumer   |                                                | - - - - |
|  Unit       |                                                |         |
|             |                                                +----+----+
|             |                                                     |
+---+---------+          +---------+           +-----------+        |
    |                    |         |           |           |        |
    +--------------------+  |   |  +-----------+  Wagobox  +--------+
     L                   | - - - - |           +-----------+
                         |         |
                         +---------+