the problem with other sources is that they do not use the same outputs. for instance the ps4 uses hdmi and optical which is digital while the z506 has analog only inputs.
instead of a surround sound set, if you went with a 2.1 computer speaker set you could grab a hdmi audio extractor box which would give you 2.1 sound from the ps4. then using a 3.5mm splitter you could hook up your pc as well.
for 5.1 analog input sets its trickier. i've seen hdmi audio extractors which pull out three 3.5mm connections (LF/RF, C/SUB, LR/RR) but they are not cheap and pull a large chunk out of the budget you listed.
if you bought a 5.1 with spdif optical inputs such as the z906 it would fit your needs perfectly. however, its over budget by a huge margin unless you buy used. there might be other sets with optical input but they are likely still high priced.
you might be able to get a 5.1 home theater in a box (htib) set which has hdmi inputs however for that prices quality is likely to be poor and it has a higher chance of not having what you need for inputs (multiples) and if it does it might not even support surround sound from inputs.
a device like this
http://www.amazon.com/Panlong-Dolby-Digital-Audio-Decoder/dp/B00JKBMXCI/ref=sr_1_9?s=electronics&ie=UTF8&qid=1400103179&sr=1-9&keywords=optical+5.1+dac which has optical inputs (ps4 side) and coax input (pc side.. or you can just bypass it and plug that side into the speakers) would work. however, it might not be available in the uk, takes up a huge amount of your budget and quality is indeterminable. to connect it to pc speakers you would need rca (2x male ends) to stereo 3.5mm (1x female end) for all 3 connectors.
if you left your pc running while the ps4 was in action its also possible to have your speakers connected to the computer and have the ps4 sound playing through them. the problem is that this requires a soundcard with audio input (probably spdif optical) which is hard to find and the cards arent cheap. expensive enough that this isnt viable for you.
in terms of audio quality the z506 really isnt bad for the price though.