MacBook 12” Battery Count: 27, Service Battery

Sounds like a random failure. It happens. I had an Apple notebook computer battery report negative capacity and "service now" status after a sudden power shutdown. If it's out of the limited warranty period and you don't have AppleCare or AppleCare+, you're responsible for it. Now with mine, that was when Apple offered a discounted "service replacement" for batteries under the rated cycle count, but that was a while ago.

The battery service price is £199 through Apple.

Mac Service Answer Center - Apple Support

Product	Battery service
11-inch/13-inch MacBook Air	£129
13-inch/15-inch MacBook Pro	£129
12-inch MacBook	£199
13-inch/15-inch MacBook Pro with Retina display	£199

I'm assuming you're in the UK since you mention pounds. There's also a statutory warranty (5 years in Scotland, 6 years in England/Wales) provided by the seller. However, that might be tricky since it's the responsibility of the seller, with the burden on the consumer to prove that there was a defect that was present at the time of delivery. If you didn't buy it directly from Apple, then the remedies might not include Apple OEM parts or Apple-certified service.

Apple Products and Consumer Laws in the United Kingdom

The Retina MacBooks have a battery that's glued to the lower case/cover. Apple's official battery service would replace the entire lower case and battery together as a single part. It is technically possible to remove the battery and attach a new one to a case. Apple sends the old assemblies to be serviced and reused with new batteries attached. I suppose it's easier to have central facilities to do this sort of thing and have the local technicians just assemble parts without needing to remove/apply adhesives.

If the seller wasn't Apple, you can't be too sure if they might say that all they'll do is an aftermarket battery replacement in the existing case. If that's the seller's remedy, then you probably won't have access to Apple's service in the future, as they consider that "unauthorised service".

My early 2016 Macbook started giving a service battery warning at 17 months old, with less than 100 cycle counts (obviously defective product). Since it was out of warranty, the Apple store quoted $299 to replace, which was $100 more than what I saw on the web. I decided not to throw good money after bad, and when my current Apple devices wear out, they will be replaced by something else.