I have to say, it does seem strange. I wonder if other European countries with LHD vehicles are also experiencing a delay to Performance Model 3’s? That would rule out any specific reasons relating to GB. Can you change Region on the Tesla website to see availability in other countries ? GB website is up-to-date showing new deliveries of Performance M3s as being May 2021.
I have answered my own question, seems like other European countries (checked France, Germany & Norway) all quote delivery month of February 2021 for Performance models, which is in line with SR+ and LR variants. Seems it is only GB that has a longer wait time for a Performance.