I have one too, also from Airsoftpartshop.
It's excellent for the price. I got mine in FDE and the color matches my realsteel MIADs better than a genuine non-PTS Magpul CTR (go figure).
For the spacers, the buffer tube on the UBR is exactly the same length as the WE and any RS buffer tube: you need to re-use the plastic/rubber spacer inside the WE's (or buy a spare one). It's in there snugly and you'll have to tap the buffer tube a few times to get it out (or pry it out with a small screwdriver from the rear).
I suppose the washers will do the job, but I don't know if that rubber spacer is supposed to absorb impact (it's that kind of material); using hard metal washers may result in damage in the long run.
Spacer in question:
