Regardless of key signature, for Handel, you should start the trill on the upper note.
Not necessarily in this case I think. But we are getting into baroque style rules and questions of authenticity.
Handel's original score does not have a trill marked on these particular notes, though there is a trill marked in a lower part at the same place.
The modern edition by Hans Ferdinand Redlich in
Hallische Händel-Ausgabe, Serie IV: Instrumentalmusik also has the trill marked on the lower parts but not the top line. (See below)
But the score used by the OP has a trill marked, and it is an ABRSM exam piece, so he must play one.
As I understand it, the baroque convention was usually to start the trill on the higher note, as
@Dibbs proposes.
However, if the trill is preceded by that higher note, then I believe that the trill should be started on the lower note.
In the full score the (non-existent) trill is, indeed preceded by the higher note. So had the trill been played in Handel's time, it would presumably have started on the lower note, like the other trills in this section. I presume all the parts would have done the trill in the same way.
The ABRSM score has changed the melody slightly so that the preceding note is not the higher one.
Personally, I feel that since ABRSM has changed the notes and added a trill not specified by the composer, we do not have an authentic baroque score and need not obey strict baroque conventions, so the OP can play whatever trill he likes.
But the important question is what the examiners would expect.
Would they care?
On reflection, assuming that the examiners are looking at the exam book, and that they care about baroque conventions, I agree with
@Dibbs that it would be safest to start on the upper note.
F#, E, F# , E