I will leave a Harrier pilot to comment on your land/hover theory but I think it unlikely. You have also compounded the issue by throwing a USMC Harrier into the mix. Where would that have come from? Why would they want to land or hover - what could two Harrier pilots do? As to the idea it was what was to hand - an RAF Harrier and a US Harrier? Does that sound plausible? Where were Harriers from 2 nations operating together in 1990? Solve that and you may get closer to an answer.
The other issue you need to address is why a late August Saturday afternoon? Trials are normally conducted weekdays unless it wasn't "one of ours". If it wasn't ours, surely it would be a QRA job, not a Harrier, especially as there is no evidence that any Harriers were airborne anywhere in the UK that day.
If you cannot account for the only two credible objects, the story starts to fall apart.