Hi guys
A Permit to Fly does not directly stop you training on your aircraft. What normally stops you is that the aircraft can not be operated for Public or Aerial work.
The ANO (Article 130) allows a sole owner to pay for the services of a pilot and to remain a private flight. This does not aplly to group ownership. However if you are in a group and have a tame instructor, who does not want paying, then the flight would still remain private and thus allowed.
When you get to the larger aircraft, over 2730Kg, there is an AIC that allows owners/group members to obtain a CAA permission to pay for training in these aircraft.
ACX