be prepared to be able to travel to one of the 7 offices that do fast track appointments. If you know when you are going back to the UK I would book on line for your appointment now. The strike finishes on the 8th May but counter appointments are still available. Go on Gov.uk to make your appointment.
Pattaya insisted on blue pen for my tourist extension last year. Two weeks ago in Patong, anything goes. They laughed at me when I brought my own blue pen and I told them about my previous experience. Blue or black pen in Patong is ok.