Fuji
You can always check the Channel water temp
here at Pevensey.
FWIW in the RN FAA we had to wear immersion suits at water temps of 14C and below, so long there was confirmed SAR within 30 mins of ditching. I forget the temp for no SAR within 30 mins, but you can reckon it would be 16 or above.
This
chart shows the accumulated sea temp data for the Pevensey buoy.
Note that only in July, August and September is the temp consistently 15C and above.