No idea ! (No G meters in Spitfires). Reckoned to be 4 or 5G (and that was a guess, I think). The Spitfire two-tier rudder stirrups were designed to delay the onset (as your raised thighs would help a bit), and you clenched in your tummy muscles, but that was about all you could do.
Hence why Bader had the advantage, he could withstand more G in combat than fellow pilots, as he had no legs to pool the blood in.... Little known fact that.
G figures are not recorded in the pilot notes I have for the Mk1 14 and 19 (Sad I know, but they were given to me) simply because without a gauge the pilot wouldn't know what they were pulling I suppose, they just give airspeeds and a warning not to over stress the wing if it has metal ailerons on the Mk1