I'll second Dawn Stars advice,If you cant make Portishead before HW dont go any further than Barry or Cardiff. No point fighting the tide above Barry. Better to wait in the Harbour then take the next flood up. You then have the option of Portishead Marina or carry on up to Bristol.

Its about 7 miles from Avonmouth to Bristol whioch takes about an hour with the tide helping. Last lock in to Bristol is usually about half an hour before HW.

No point fighting it