That file is a mess. It looks like the number of spaces around the numbers is significant, which is never a good idea.
Thankfully for you readmatrix seem to import the file correctly. It also detects the number of header lines on its own. All the importing functions (csvread, dlmread, readtable, readmatrix) allows you to specify the number of header lines if you need to. The last two can detect that automatically, so:
data = readmatrix('Full_beam-OAM_plate_l= 1_Intensity.txt')
