We first noticed these flat peaches a couple of months ago in one of the grocers or small supermarkets in Coventry which specialised in foreign foods. Shortly afterwards we started seeing them everywhere, including in Tescos.
They seem to have come from nowhere and become ubiquitous very rapidly. We bought a pack last year and were dismayed to find they went mouldy very quickly but I did manage to salvage one to eat. It was quite juicy but slightly bland. I think I will stick to normal peaches or nectarines.