Pooled Fleet
Pooled fleet means the RAF aircraft will enter the upgrade and maintenance PDM flow as part of the overall RJ fleet. The RAF will only be flying the three designated airframes, 664, 665, and 666. This means two aircraft will probably be available at any one time for ops, not three.
Although L-3 has tremendously improved the time required for PDM and Block upgrades, usually a third of the overall RJ fleet is in Greenville at a time. The flow chart of this looks like a cascading series of PDM periods with the individual RJ aircraft coming through about every three years. This has worked to keep the fleet upgraded with the latest aircraft operational and mission equipment since the 1970s, and should be good enough for the RAF.
Unless, of course, the whole certification disaster discussed in the previous 34 pages of comments screws the whole thing up. Biggest loser will then be the RAF, MOD and the UK---left without a premier airborne SIGINT platform.