Shipping isn't usually an issue...the BIG Car transporting stuff bound for Portishead is EASY too see,slow and easily avoided..and weekends they are mostly at anchor anyway.
You probably know Porlock is best 2 hours either side of high for an easy launch.
I'd definatly recommend going for neap tides to help negate Tidal flow...and Races.
Personally I'd be thinking of going down to Lee Bay anyway if I were you