Looking at roundtrip business class award trips from the US/Canada to Europe, it appears that one gets hit with the high ex-US BA surcharges twice if you book a roundtrip from the US to Europe, while if you book it as two one-ways, the surcharge from Europe is much lower.

US-Europe-US booked as a roundtrip results in US$1,600 in BA surcharges (2xUS$800) while booking two one-ways US-Europe and Europe-US results in about US$1,000 (US$800 for US-Europe, US$200 for Europe-US) in BA surcharges.