The only reason is finnacial. With Ark's recent re-fit (essentially giving her the ability to act as another "Ocean" as gunnut suggested) and with the demise of the SHARs, basically the Navies carriers are glorified LPHs. Sad, but practical, in some ways.
I've posted on a realistic re-model for the RN before but will do so again (just because I can't help it
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2 CVF - 27 JSF per hull, CATOBAR with the "C" variant of the JSF. 4 Advanced Hawkeyes, plus ASW helos etc. Wartime, able to carry far more JSF as necessary.
2 LPH - Ocean and Ark and then two larger two successors. Able to operate "B" variant of the JSF at need.
8-12 T45 (but re-fitted with a LACM capable v62 cell VLS (i.e A70), plus AsuMs, torps, Merlin capable, 155mm gun, Hangar mounted RAM launcher).
12 T23s with Sonar 2087
4 T24, basically a lengthened T45 without PAAMS/Aster 30, but with Aster 15 and LACMs, 155mm gun, 2 helo hanger plus flag capabilities (to replace T22B3).
Hangers retro-fitted to Bay class LSLs to make them multi-mission capable - acquire USVs to operate from them. Oh and buy 2 more them.
8 enlarged OPV(H) River class with increased endurance. Hangar to take two Flynx or 1 Merlin. Fitted with with 4.5 inch gun, 2*30 mm. Fitted for (but not with) torp tubes, AsuMs, 16 cell VLS, Sonar 2087. Able to carry a platoon sized EMF.
8 Astutes.
3 Astute variants with 8 VLS tubes for either a Trident successor or SLCMs.
The OPVs take up the jobs that frigates are currently performing, and do the single ship, presence missions. The enhanced and increased Bays give a greater versility to the RN and the T24's enhance the RN's deep strike capability.
An FSC program would eventually replace the T23's - and should be a large (9000 tonne plus) trimaran design, with an emphasis on ASW, land-attack, MIW and able to operate USVs, UUVs and have area defence AAW capabilities.