Optical out complicates things NUC's dont normally have those but you can buy HDMI audio extractors like this
for £26. One other option is a USB sound card which have SPDIF optical out like this for £9
but I've no tried it personally, in theory it should work but the Openelec forums may know better.
Celeron NUC £103
You dont need to buy a mSATA drive you can run Openelec from a USB flash drive if you have one lying around. But if you want one here is a mSATA for £22
Lastly a remote, get a cheap MCE remote for £10-15 or get the Pulse Eight CEC adapter (around £30) if your TV supports CEC and most should these days (edit forgot about audio extractor the PE CEC adapter may not work with that in place so you might be limited to MCE remote or use a tablet/phone to control XBMC).
So with extractor, NUC, RAM, MCE remote you're around the £160 mark, still cheaper than things like the Popcornhour and more capable.
It doesn't look like the Asus Chromebox is sold in the UK, all the prices on Amazon UK are very high import prices.
Another XBMC option with optical out is the Solidrun CuBox-i
series a more powerful version of the Raspberry Pi, this has optical, IR and HDMI-CEC out of the box, all you do is buy the unit and a micro SD card, burn Openelec to SD card and then put in the box and turn it on. Get the i2 (dual core CPU) or the i4 (quad core CPU) model, the downsides are support for CuBox-i is still in alpha state and they make them in batches, due to high demand there is waiting time in getting one.