Ah, that would be the changes described
here on National Rail Enquiries, a closure between Bristol Temple Meads and Westbury. It's very disappointing that SWR have yet to reflect this closure in their timetables - let alone in their own planned engineering works page - and that they are
still letting people book Advances on trains that they (ought to)
know cannot possibly run. I would go so far as to say it is highly irresponsible and represents a very lax attitude to delivering on what they say.
As stated - if you insist on getting to your destination on time then your only option is to leave earlier. The good news is that, since replacement buses aren't reservable, you will be able to take an earlier bus than your originally booked departure time, and one hopes that you will then be able to pick up the original train at Westbury, but it's anyone's guess as to whether or not the train will run normally from there.
Alternatively, if you have yet to pick up your tickets then, if you do so at an LNER ticket machine, this will only print something along the lines of "Valid on reserved services shown below and required connecting services" on it, and then the booked service (without evening mentioning the company). There is nothing on there that shows that it is an SWR Advance ticket - and so I don't see that a GWR guard would have any basis to object to you using their services, because there is no company restriction printed on the ticket (and, though less relevant, they both have a common parent company - First - so it would seem the "done thing" to arrange ticket acceptance).