The best way to buy speakers (and in fact any true hi fi kit ) is to listen to it.
Argos are great at shifting boxes of stuff but I haven't yet seen an Argos with an audio demo room.
Try richer sounds but as they are a discount place you will neeed to check first if their listening room is available and they have the kit you want to listen to.
http://www.richersounds.com/home.php?cda=front page
Take your own audio cds with music you know well. You may be surprised how different they sound on various combinations of amp / speaker.
Other places you can try are larger currys stores (only some have proper listening rooms )
Zavvi ( I know of a couple that have a demo area.
Dixons in most high streets will probably have a hifi rigged up to listen to but not in a quiet environment.
Your local inedpendent hi fi store will also be happy to demo speakers for you but will probably need to book first so they can set things up.