The Amex Plat travel agents have done maybe 3 or 4 GUFs for me, without much issue. They normally need an extra 5-10 minutes over a regular flight booking, to check some details with a colleague or look up the instructions on their intranet, but otherwise have been fine. No explicit additional fee for the GUF / booking fee (other than the 450 quid a year for the plat...), but IIRC lately it's something like 25 quid more than the BA website / ITA Matrix shows for the same flights, which I believe is an NDS vs GDS surcharge from BA.