JTO, yes but the plan for a long time has been that UK Pilot and Maintainer trg ceases in the USA in 2019. This was a condition of the Pooling Agreement with the USA. By 2019 the bulk of personnel will already be trained and 617 Sqn will be around 140% of its final size - this will grow to around 170% by the early 2020s and, approaching 2022, a shadow 809 will likely form as seed corn; the remainder being provided, business-as-usual, by the OCU and ITC at Marham between 2021-2023.
Some of the older RN cadre who have hung out in the USA, and who don't feature in the 617 Sqn or OCU manning assumptions, may we'll be too early for 809; as you say. RAF saw that with Case White (Typhoon) personnel.