Perfectly valid.

The restrictions on that fare, the Off Peak Return (return valid for 1 month) can be read here:
http://www.brfares.com/#faredetail?orig=TAU» &dest=EXM&rlc=SRN&tkt=SVRThe relevant part of the text for your journey is:
Tickets are valid on all trains timed to depart at or later than 0833
However, if it's a day trip to Exmouth, returning to Taunton later the same day, then there's the cheaper Off Peak
Day Return:
http://www.brfares.com/#faredetail?orig=TAU&dest=EXM&rlc=SRN&tkt=CDR▸ The relevant part of the text is:
Not valid on trains timed to depart before 09:00