This has me a little confused.
'#4 Mk2, UF '55' is a Fazackerly made Lee Enfield #4 Mk II rifle from 1955, I have a similar rifle from the same UF '55 lot, but with an A 189xx serial number, very close but no cigar, but not an envoy or enforcer, it is chambered in .303 British, the classic Lee-Enfield load. There were versions of the Enfield as dedicated marksmanship rifles, the L39 & L41 but I'm not familiar with the markings these would have had on them, I'm assuming it would have been marked as L39, or L42 respectively, not #4 Mk II???
However the ammunition headstamp is for a 7.62mm NATO load (the L42A1), not to be confused with the L42A1 designation of the Envoy. The crosshair (cross in a circle) denotes NATO spec ammunition BTW. There may also be a green, or black spot (filled in circle) on the box.
Could you have a #4 Mk II rebarreled to 7.62mm? This was a popular conversion 'back in the day' & was used a lot for long range (1,000 Yd) competition.
Is there a caliber marking on the barrel itself? this should clarify the chambering of your exact rifle & remove confusion. The 7.62mm barrels were a lot thicker than the original .303 barrels, but this was done to handle the higher pressure of the 7.62mm load, not for anything else.
As a hint. The Lee Enfields had wood almost all the way to the muzzle, only about the last 2' of barrel being visible, they also had a wooden covering over the top of the barrel to the same point in the length. The Envoy & Enforcer rifles had a much shorter front stock which stopped about 1/2 way along the barrel length & the top of the barrel was exposed all the way to the breech. Of course this may have been changed as part of a conversion to 7.62mm.
This is my Lee Enfield #4 Mk II.
The sight, mount & cheekpiece have been added to a stock rifle by me, but you get the idea of what a #4 Mk II looks like in .303 Brit.