These are what I buy for myself and others most of the time:
http://svp.co.uk/product/verbatim_si...3550)_3798_box
These are all excellent choices too:
http://svp.co.uk/product/verbatim_dv...3548)_3398_box
http://svp.co.uk/product/verbatim_16...3548)_3652_box
http://svp.co.uk/product/verbatim_br...3548)_3787_box
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Price are PRETTY MUCH IDENTICAL to what it costs in the USA or Canada. It's about $15-18 for a 50-pack, or $70 for a 200-pack. To spend any less money will get you crappy discs from CMC, fakes, RITEK, etc. With cheaper media, all you save is maybe $5, but you end up with tons of coasters and discs that must be replaced. Verbatim tends to burn 100% perfect all the time.
For your £20, you can pick up 2x 50-packs, so 100 discs. That's a good price. And those are archival-grade discs.
Use them for special needs, such as fellow CPS members, your own masters of sets, etc. Keep the cheap ones for people you don't like, non-CPS, etc. We like the good stuff here, we've got higher expectations.