If it came from the factory that way, or is a Jaguar-approved add-on for that model probably OK
If an aftermarket addition, and it seems from your question it probably is, probably not
Gone: '59 Mark I, '56 Mark VIIM, '59 Mark IX, '01 XJ8 Sovereign 4L LWB, '01 XJ8 Sport 3.2L, '01 XJ8 Exec 3.2L
Current: '03 Strange Rover L322
In trim shop for total refurb: '73 Owen Sedanca 4.2L
Stalled in storage '61 Mark IX with 4.2L
It didn't come from the factory like that so it's not official. It will depend on how far back from the leading edge of the bonnet it is mounted and more relevantly, what frame of mind you catch the MoT tester!
1995 XJS 4.0 Convertible
1980 XJ-S Pre-HE
Jaguar 4.2 Supercharged engine (but not with a Jaguar body..)
I have had spring loaded leapers on my last 2 X Types, I checked with the local police, they told me if it passes the mot then it is legal. So 8 years later I have never had a problem. Good luck.