It was the JP5a which had them. The 5s at Finningley used for Nav training didn't. The 5as also had the roughened leading edge which was also part of the fit to make the spin more stable.
I'm not sure exectly how the strakes worked, but the main difference between the 5a and the 3a was the large canopy on the 5a. Also, the 3a had tip tanks which the 5a didn't (except for the Nav training Mk5s). I expect it was something to do with these facts. What I do know is that the 5a burned 5lbs a minute more at low level than the 5.
And I'm not sure they worked that well. Spinning a JP5a one day was the closest I've ever come to parachuting!