Its usually about 50 quid for the battery alone. Dealer can quote you around 200 for a full service- quite ridiculous. I found the number for Breitling UK direct in the literature that came with the watch. They were extremely helpful and courteous, and some off-warranty work was even completed on good will as they were a little embarrassed about a malfunction...
Also, there is a guy who goes around as a military rep and he is very good at getting things done, and the bills are less than you pay at Goldsmiths in the High St. Can't remember his name and contact details tho, perhaps someone else knows him?
One tip tho- whichever method you use (and I wouldn't advocate the watchsmith in the market by the way) watch them like a hawk throughout your dealings. They tend to rather rely on their over-promoted reputation than actual quality workmanship. This has previously manifested itself in such situations as returning a watch to me having had a replacement battery, only for the battery low indication to be displaying as the (embarrassed) jeweller handed it back to me after already swiping my credit card!
Personally, I don't rate the product or the services any more, and I think they merely trade on perceived quality rather than actual. But having already bought the thing I advise you get it repaired/serviced properly as this is the only way you can moan at them when it all goes pear shaped!