A single from Birmingham New St - Ashchurch for Tewkesbury would be needed.
As one of your tickets is a Rover you don't need to travel on a service that calls at Ashchurch for Tewkesbury.
Condition 14.2 of the National Rail Conditions of aTravel:
If you are using a Season Ticket, daily Zonal Ticket, or another area based Ticket such as a concessionary pass, ranger or rover in conjunction with another Ticket and the last station at which one Ticket is valid and the first station that the other Ticket is valid are the same, then the train does not need to call at that station for your combination to be valid.