This is a surprisingly difficult question to determine the answer to... parks seem to love providing positive g-force readings for rides, but rarely provide negative g-force readings for some reason.
The best way would be to get someone to ride every single UK roller coaster with an accelerometer, but that would be very time-consuming, and possibly very inconsistent, what with variable weather conditions and such able to cause considerable variation in g-force levels...
This is such an interesting question, yet also such a difficult one to find the answer to, in quantitative terms!
P.S. Sorry for double posting!