As above, only option for 5.1 with optical are the Z906's. £350 on their own site is too much. OcUK have them for £300, but have none in stock. They can be had for £250. Quite a well known place and should be easy to find.
Stereo speakers with optical are pretty slim pickings as well. It's easier to get an SPDIF DAC for around £10 to which you can add any powered stereo speakers you like. That way you're not tied down to a very small handful that have SPDIF connection built in, such as Q Acoustics BT3. Good speakers, but depends whether you want to spend £200.