The minimum engine size for a motorbike on British motorways is 50cc. A Trippi and a so called Nippi 50 are only 49cc quite deliberately. However there are two other things to consider also.
Regardless of the engine size you cannot ride a Nippi or Trippi on the motorway on L Plates (you need to either of completed a CBT in a previous life, because you cant do a CBT on a trike, or hold a full car license issued before 2000.)
If the Police come across you and consider that you are not going fast enough and endangering yourself or other road users they can order you off the motorway regardless of whether you have the proper license and engine size!