A TA would have access to the BA sales prices, it just means that instead of needing to find award inventory to apply the GUF2, a TA can just book you in the upgraded class assuming there is A/I availability (F/CW).
If you search for GUF2 you will find many threads describing this. The difficulty is finding a TA that can do it.
Thanks! I've just done all bookings on BA.com, so seems like I need to find a good TA I can use! Would it work the same way if I just called BA, or do you get better fares by using a TA?