Not strictly true JDJ. As Flight Levels are based on the pressure a fixed datum of 1013mb (HPa), if the atmospheric pressure is high enough then you can get FL025.
If my maths is correct: (neck on the line here ;-) ) with an RPS of 1047mb , as you climb and level at 3000ft RPS and set 1013, your altimeter will read ((1047-1013)x 30)=1020ft less than it did before ie 1980ft.
Rounding up to the transition level of FL020, add 500ft foot to get Minimum useable flight level of FL025.
Extremely rare but it can, and has, happened.
(I do hope I remembered that correctly, it is some 14 years since I was last given a pressure question)
(I make the assumption of 1013 and not 1013.25, and that 1mb is roughly 30ft)
A lot of the spotters "radar tools" tend to have a single datum on the display and report all heights/altitudes/flight levels in the same format.
Back on topic, I believe there are 13 Gripens operating out of Leeming and taking advantage of our facilities, certainly Donna Nook and a couple have flown high through the Lake District.