Model runs over the past week have suggested anything between 13.9*C and 15.9*C to be possible so that's helpful 
Seeing a consistent signal for a lot of warm air to move NE from the N. Atlantic toward the UK so even if it's mostly via balmy nights I can see plenty of relatively high CET returns for the middle part of the month at least.
Enough that I'm prepared to be a little bold and go for 15.4*C.
Hopefully September can recover 2017's reputation a bit after such a shortfall relative to the consistent signals back in July for some decent warmth on multiple occasions in August. 
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https://twitter.com/peacockreports 2021's Homeland Extremes:
T-Max: 30.4°C 21st Jul | T-Min: -6.8°C New Years Day! | Wettest Day: 34.1mm 2nd Oct | Ice Days: 2 (27th Jan & 8th Feb)
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