This may help clarify the situation. Extract from a old Janes article on BAE Systems 120mm Tank Ammunition.
"XL23E2 APFSDS Projectile
In November 2006 the UK Ministry of Defence (UK MoD) announced that it was considering a programme to develop a 4th Generation tungsten projectile under the designation L28A2. The contract would be issued to BAE Systems, Global Combat Systems Munitions and would cover fabrication and test of prototypes up to, but not including, the design chill and qualification phase. They explicitly noted that this was not envisioned as a replacement for the DU-based L27 series ammunition used in the Challenger 2. The designation was therefore changed to L23A2 as it is seen more as a replacement for the first generation APFSDS L23A1 shot. British-funded firing trials to demonstrate terminal ballistics have been carried out. British qualification had been scheduled for 2009, but was held back to 2010, after which production for Oman began. The projectile will be backwards-compatible with the older L11A5 gun found in Jordanian Challenger 1s.
The XL23E2 shot is to replace aging Omani stocks of L23A1s, and may also be procured by the British Army for use in lower-threat environments that do not require the capabilities of the L27A1 DU projectile. This takes advantage of some work done on the L28A1 developmental projectile but features the lighter projectile weight (7.95 kg) of the L23A1 and integrates with the C3TR L18A1 propelling charge. The use of the C3TR propellant extends barrel life but reduces muzzle velocity. Due to improved penetrator design, external and terminal ballistics are still better than the L23A1, but decisions are still pending on final design and production. "
Hope this helps.