if you can find a 216 sp 260 for around £135, then you'll find the 285 is OVER 80% more expensive, yet, what, 15% faster at best, when both maxed out overclocks, maybe a smaller difference as the 216 "should" have more headroom for overclocking starting out at a lower clock. if they both hit the same final clock(not sure if they do, no idea how either clock) then the difference is likely to be negligible.
Personally i think the 275 for £200ish is a very very hard sell considering the 260 is so damn cheap, another £50 for another tiny performance bump is just as unsellable to me.
Keeping in mind you'll be almost at sli 260 costs to get that 285gtx, frankly, 260 sli spanks the 285 just as hard as a 4870x2 spanks it left right and centre.
I can't think a of a single situation where playing 80% more for 10-15% performance increase is good value.
The only valid reason(and i only just noticed you have a 4870x2 now) to downgrade to a single card solution is to make money going with something cheaper because you need it, or like I did, I went to aa 4890 because my room is, just, insanely hot in summer and especially at the moment mainly playing windowed mmo's running an extra core, an extra 100-120W all the time for no reason in a baking room isn't worth it at the moment.