Doesn't really work that way. We currently have three flat-tops on a cycle which means we have at least two front-line at any one time. To keep two fully active requires three, since a lot of the time one will be in refit or working up. If Ocean is kept in permanent reserve, we would sometimes only have one CVF active, & no LPH.
What I meant by the term 'reserve' is that in a protracted conflict scenario Ocean could remain in UK controlled waters on stand-by to relieve one or both of the QE's deployed. Once the high-tempo assault operations are over and the RN deems it appropriate to maintain only a command & control / hospital / technical support function, then Ocean can step up to the plate.
As we witnessed during the South Atlantic conflict in 82, the capital ships can and will take a real battering in heavy seas over a protracted period, hence once the initial beachhead is secured why not then retire the capital assets for maintenance (wear and tear / battle damage) and replace them with Ocean as the primary command and control ship? A flight of F35B's would provide CAP from improvised airstrips on-land and flown clean to Ocean for maintenance.
If all goes according to plan the RN could potentially rotate two ARG's one after another in a staggered format, and still have fat (Ocean) for extreme emergencies, as follows:
Alpha Deployment (Active ARG - 5-days notice to move)
1 x QE (Strike & LHP configured) mix of F35B, Apache, Merlin, Sea King and Chinook
1 x Albion complete with 4 x Mk10 roll-on, roll-off LCU (each capable of deploying a mix of Viking and Challenger if required). UK currently has a total of ten MK10's in service.
2 x Bays (plus MK5 LC and / or air-cushion)
2 x Commando Units (45&42) plus 2 x SF sqn & 2 x companies of SFSG, artillery (2 x battery 105mm from 29) / eng / armoured recce / support (Viking / CVT / tracked Star-streak) split between Albion and Bays
2 x T45
3 x T23 (CAMM upgraded)
2 x SSN
2 x RFA/MARS
MCM as required
Bravo Deployment (Reserve ARG 30-Days notice to move -T45's on global deployment recalled to fleet).
1 x POW (Strike & LHP configured) mix of F35B, Apache, Merlin and Chinook
1 x Bulwark complete with 4 x Mk10 roll-on, roll-off LCU (each capable of deploying a mix of Viking and Challenger if required).
2 x Bays (plus MK5 LC and / or air-cushion)
2 x Commando/Army Units (40&1Rifles) plus 2 x SF sqn & 2 x companies of SFSG, artillery (2 x battery 105mm from 29) / eng / armoured recce / support (Viking / CVT / tracked Star-streak) split between Bulwark and Bays
2 x T45
3 x T23 (CAMM upgraded)
1 x SSN (adding to 2 mentioned above, possibly leaving 1 to return or to be reassigned)
2 x RFA/MARS
MCM as required
Extended Reserve, 60-days notice to move (Alpha ARG retires for repair / refit, ER backs up Bravo ARG now on station)
1 x Ocean (Puma & Apache)
1 x Argus
Chartered Commercial Vessel (16 Air Assault Brigade built around 2 Para* / 7RHA / 9-Sqn Eng & armoured recce troopers who would man kit brought in earlier on the Bays and now ashore)
1 x T45
2 x T23 (CAMM upgraded)
Chartered tanker support and roll-on / roll-off vessels
*Note: This would leave 3-Para as the UK's designated rapid response unit on 24-hours notice to move should another emergency occur. The remaining UK Land Army/RAF/Armoured Corp et al would continue to rotate through A-Stan should by 2016-18 it's still running hot. Bronco's would have replaced Vikings in theatre, and with the purchase of Viking II's already approved the Commando Brigade would be back to full strength with regard to amphib APC's. The Bronco's in A-Stan would be crewed by RTR personnel leaving the RM to focus on there own Vikings.
All above based on the assumption that both QE's built, 50 F35B's in service and 6 x T45 are now commisioned (one in refit). Also NATO allies step-up and take over RN's obligations to standing patrols as was seen in 82.
If one of the QE's is in refit and not on its scheduled training cycle then Ocean would have to step-up as the primary vessel in the Bravo ARG. The second QE would then be rushed back into service and travel to theatre with the extended reserve.