I had a quick look and I found Centaur 10s 12-27 in stock at Merlin. Cost is about £80.
Wiggle have the cheaper Veloce 10s cassette (in most ratios but not 12-27) for £37.
IIRC 12-25 and 12-27 only differ in the largest two sprockets, so if these are not used much and don't wear quickly, you might be able to convert a 12-25 to 12-27 by re-using some of your old sprockets.
I bought 3 Centaur 12-27 cassettes (one for the bike, two spares) from various places, think the most I paid was £69. They're getting a bit hard to find now, though, as I think they may have recently been discontinued.
These are the 10-speed Campagnolo cassette ratios I'm aware of:
Campagnolo Centaur Cassette Ratios
• 11-12-13-14-15-16-17-19-21-23
• 11-12-13-14-15-17-19-21-23-25
• 12-13-14-15-16-17-19-21-23-25
• 12-13-14-15-17-19-21-23-25-27
• 12-13-14-15-17-19-21-24-27-30 (30T is apparently too large for older derailleurs, though)
Campagnolo Veloce Cassette Ratios
• 11-12-13-14-15-17-19-21-23-25
• 12-13-14-15-16-17-18-19-21-23
• 12-13-14-15-16-17-19-21-23-25
• 13-14-15-16-17-18-19-21-23-26
• 13-14-15-16-17-19-21-23-26-29
The Centaur cassettes are a bit nicer quality, they're a bit lighter, have a different surface finish and the last three (I think) sprockets are on a carrier and less likely to gouge the freehub (though that's harder to do on Campagnolo than Shimano freehubs anyway)