As a brother of one the afformentioned Harrier pilots and bimbling around similar airspace, he gave me stern advice - stay above 2,500ft on weekdays if you want to be safe
I don't know how many days of the year you can actually get up to 2,500ft VFR in Devon, probably not that many.
The probability in our extensive uncontrolled airspace here of hitting a fast jet on its way through seems very low. There's a lot of space. It's not like there's a narrow gap that everyone has to squeeze through and they only come over about once a day, sometimes twice
In the last analysis, you have to accept a level of risk in life. I think we get too hung up on safety at all costs sometimes.
QDM