Sandiego89;
The 'Ramp' at Yeovilton was dismantled not long after the Sea Harrier was decommissioned. The run-up is still there, as is the 'pit' but the actual ramp itself has gone.
I can't say for definite but I think the Wittering ramp has also gone.
A couple of years back I attended a RAeS lecture by Doug Taylor ( ski-jump creator) and he said that the US tested virtually all their fast-jet aircraft at that time on the Ski-Jump.
He showed some slides of various aircraft 'jumping' and, from memory, included A-4 Skyhawk, F-5 Freedom Fighter and, I think, an F-4 Phantom.
I believe the trials were successful at the time.
I never could understand why the US Marines never fitted a 'Jump' to their ships. I'm told that whenever the USMC cross-decked to one of our carriers they spoke very highly of the Ski-Jump.