The Ti-HD (Titanium-HD) is great for headphones and a 2-speaker setup, but not designed for directly working with a 5.1 (6-channel) setup.
The Ti-HD's optical output can be used to connect to an 5.1 speaker setup. but your not budgeting enough for a "quality" 5.1 optical connection setup.
Is is possible to buy a used receiver with a 5.1 speaker setup (200 GBP?) and use it's optical with the Ti-HD, yes, but the optical connection bypasses the really nice PCM1794 DAC chip and op-amps that come built into the Ti-HD.
Personally if i was you, i would buy some nice stereo headphones and plug them directly into the Ti-HD, maybe spend 50 GBP for a pair of self-powered speakers.
The AKG K612 Pro (120-ohm) headphones (110 GBP) should work nicely with the Ti-HD.
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http://www.thomann.de/gb/akg_k_612_pro.htm?sid=3dcfbefa350c3585c43feb14cacc9da9
The 16-Ohm Heaven Cs are really designed for use with portable audio (smartphone/mp3 players/iPods/iPhone).
You could spend 80 GBP for a headphone amplifier, like the EPH-02, plug it into the Ti-HD and the Heaven Cs into the EPH-02 and it should improve the audio quality, but I think that would not be the wisest choice.