Cracking bikes are GS, i'd look at getting it from BMW dealer, yes price is higher, but their warranty (on bike side) are extremely impressive and include full European breakdown cover. Its worth it, as 1200's are known for final drive failures (which is covered under warranty)
Consider the GS, over GSA, i need to rest my arse or fuel myself long before i need fuel, so 300 mile range of GSA is not used, its also lot heavier, GS is lot more flickable in the corners. Also £2k cheaper! 
Your welcome to take mine for a blast 
Had a testride on the GSA few weeks back down with the BMW offroad school , by test ride though it was more like 3 hours on dirt tracks and the odd normal road

Bloody loved every minute of it!
Reason I want the GSA is purely for the tank range, as I'm planning on doing some stuff with globebusters when I get it, tempted with the silkroad trail! And sitting on a bike for hours on end is no bother to me, I took a slightly elongated route back from keswick today... Left at about 8:30am, and got home just before 4pm, only stopping for fuel (literally, stop, fill, pay, go). About average for a rideout with me that is, longest I've done though is 600 miles in a day from John O Groats back to mine (Northwich) without touching motorways... Was fun

I did have a go of the F800 and F650, but they just didn't feel right with me on, the GSA though, was very snug! Even lest the ESA on its mid-height setting with no bother and could easily touch the floor with the standard seat height too (which alot of other folk had bother with)
If the offer is there though for a blast on yours, make sure you have some nobbly's fitted haha!