Late September to early December we have the Polar series at Stone.
2 races back to back 10.30am start, think is £5 a day for visitors or £20 for series and no charge to leave your boat.
You won't find a better shower after sailing anywhere this winter!! :-)
In January theres the icicle race at Mayland, a one off mass start handicap.
Then every other weekend from early Feb to late March Blackwater sailing club have winter series, which iirc is £5 per day
If you don't want tidal estuary sailing then Alton is the best bet
*edited to add*
None of the above are 'official' Eastern region Phantom events, just suggestions