I haven't found much options online with discs - just OEM ones from dealers or suppliers online, but price is pretty much the same - reply if you find cheaper as I need mine doing soon - only UK online ones I found were EuroCarParts.
You have various suppliers for pads from EBC green stuff etc, to Pagid and ATE from Euro Car Parts, eBay or other online suppliers else try CA Automotive - and they are pretty much cheaper than OEM - but you would have to search this forum for the the benefits.
Just had my rears done at dealers and it was expensive - but expected with 370mm discs - well...expected with a BMW Dealer full stop.
Quote from Benham in Wolves for supply only inc VAT for fronts was...
Standard Front Pads £217 (both sides)
Sensors £20 x 2
Discs £290 x 2
Once you ask them to fit it I would estimate £1000 to get it all done in one go as I would get them to change the brake fluid as well which would another £100 I guess - but this is less than a BBK which unless anyone else shouts out is the only other option bar getting your own.