Rigga - it is not illegal for foreign civilians to fly 'our' military equipment. The pilots just have to be approved by the MAA (this activity for test and evaluation used to be carried out by D Flying/Flight Test Div which is now subsumed into the faceless animal that is the MAA).
This aircraft is F-regd as the military EC(Fr) activities at Marignanne is regulated by the French mil regulator and requires all aircraft to have a F-reg. The first 2 aircraft as I recall we be constructed and tested at EC(Fr) with the rest at EC(Ro). EC(UK) will put the UK 'eyes only' equipment on board and test and provide RTS recs.
For EC(Fr) activities, the UK serials should be 'clearly obscured' as they eloquently put in their regulations however this does not appear to be the case here - ho hum.
When the aircraft are at EC(UK) they will be operated under the UK mil reg as this activity now comes under the auspices of the MAA.
Clear as mud eh?