A 19°C September isn't technically impossible, it is exceptionally unlikely - and my point was that it will certainly not have "200mm of Atlantic Monsoons".
December is a different story entirely - sustained advected tropical maritime winds in December can create very high temperature anomalies compared to what the same synoptics would produce in September. That said, December 2015 was the first such month in 350 years of records. Could be that long before a repeat of the month. The Atlantic's effect in winter is to keep the UK milder than further inland areas, and vice versa in warmer, summer season months, where it has a more moderating effect. This is basic common knowledge. Hence why, as I said, a 19°C September requires a lot of hot southerly winds, not "Atlantic Monsoons".