Hawk T1 OSD is scheduled as follows:
. 736 NAS - 2020
. 100 Sqn - 2027
. Red Arrows - 2030
The 736 NAS Hawks will be replaced from 2020 by the ASDOT (Air Support to Defence Operational Training) contract, which will also replace the Cobham FA-20s, and, in time, the 100 Sqn Hawk T1s. ASDOT will be an outsourced contract, with Qinetiq/Thales already having bid with the Textron Scorpion.
So any new Red Arrows A/C won't come until the 2030 timeframe.
However, by 2030 the existing Hawk T2s will be getting a little long in the tooth as lead-in platforms to F-35 and Typhoon, especially since Typhoon is expected to have undergone a cockpit upgrade in line with F-35 by then (including a single large-area display, and HMD to completely replace the HUD). In addition, all the twin-stick Typhoons will likely have been retired by 2030. So sending students solo on F-35/Typhoon, having come straight off of a Hawk T2 that is no longer operationally representative, is not a great idea.
BAe are already working on updated Hawks, which will include a large-screen display (pictured below). Other features such as a HMD, AAR probe, targeting pod, and practice weapons should also be included in order to download more training from the OCUs and better prepare students for the front line.
So the best option, in my opinion, is for the RAF to order a new batch of Hawk T2s in the mid-2020s to be used for training from 2030 onwards, and give 11-12 or so of the existing 27 Hawk T2s to the Reds (and retire the rest).