I am continually ‘surprised and delighted’ by the service I get at an Apple Store (as is their actual aim, or at least used to be) - this one Regent Street.
I’ve noticed some weird battery & charging behaviour with my iPad Pro 9.7" (7 months old, bought one at launch but an Apple update bricked that one so this was a replacement 
):
- Charging only up to 92%
- Sudden battery level drops
- Inconsistent charge times
I have AppleCare, I arranged a call and they spent plenty of time asking me about the problem - they ran their remote diagnostics and set out their plan -> reset to factory settings via iTunes, set up from new, and then if no improvement an Apple Store appointment for an engineer to run more tests.
The reset didn’t work, I called this morning, booked an Apple Store appointment for this afternoon (must have been a cancellation) and was seen straight away, some minor tests run but basically I was out of there in 10 minutes with a replacement.
Superb - and they almost tempted me into trading it in there and then for the new iPad Pro 11"!