Usually you will find that you can operate in this way in order to repatriate the aircraft and crew back to a base where that cabin crew member can be replaced.
Even if you were Easyjet flying a 149 seat 737 you will probably find that they can do it aswell, however there are a couple of restrictions.
1.Of the remaining 2 cabin attendants one must be a CA1/No1
2. A cap is put on passengers so that no more than 100 are carried. (keeping to the 50:1 ratio)
If you had less than 100 pax then this scenario seems perfectly fine and if it appears in your ops manual then the CAA must be fine with it too.
Hope this helps
Jobs!