I went to parkway to change on a via Stroud ticket? I guess I am not meant to be?
We (at TransWilts) have had some interesting discussions with
GWR▸ and established that on principle passengers should have to pay no more for their journey when engineering works are going on or there's other disruption than when trains are running to the normal timetable that's published twice a year.
As a "for instance", passenger on the 11:32 from Warminster for Chippenham via Melksham - change at Westbury, arrive 12.12, normal day return fare £7.50 should be allowed to go via Bath Spa rather than having to either pay £14.00 or not arrive in Chippenham until 14:45.
This makes common sense - passengers should not have to pay for the failure of the railway industry (for whatever reason) to run their regular services ... and for diverting them to a more expensive route.
In practise, I did not hear of anyone being surcharges during the first (March) period of the change above - but as we have so many engineering days (over 50 this year) where the same thing applies, we persisted and got the principle established. It has also helped passenger who's evening train was moved a few minutes earlier (and then took longer!) as the start time moved from super off peak to off peak.
If there's changes such that you cannot practically travel the normal "via" route, then I believe that the principle we have established allows you to take any route that's reasonable on the day. I will leave it to you to judge how that works today ...