I guess that they can't use the modern fire engines as everytime they were handed back to the fire fighters the appliances would have to me made u/s until all the systems were checked out.
When you use a hire car, you tend to mis treat it as it's not yours. You might run the engine ragged or wear out the brakes but you don't care and then hand the keys back to the next user.
You can't do that with a fire engine. You don't want to be given the keys back from the forces personnel and go straight out to a shout not knowing if the equipment will work or not!
But then again they shouldn't be going on strike and this is all down to the political spin doctors at number 10 calling the FBUs bluff, forcing them to go on strike and making the whole nation suffer so taking the heat off the bad press this goverment has been getting recently.