On 10 Jul, 23:28, Steve wrote:

That's not the only way! What about changing at Exeter St. Davids?

(or Bristol TM, or Bristol TM and Weymouth, or even Reading and
Basingstoke...??)

There's a rule which says that you cannot pass any station (where the
train stops) which buying a ticket to would cost more than buying a
ticket to the destination station. The doubling back is a deviation of
this rule. It stops people buying a ticket from Birmingham to London
via Edinburgh, even if the same track isn't covered twice.

STD Single, PAD CLJ = £5.10
STD Single, PAD RDG = £15.60

Therefore going via Reading is not an option, NRE says you're expected
to use the tube.