It is 75M from Lundy to Portishead, so you will have to punch the tide for at least half the time, and be averaging 6kn+ through the water. Don't bother!

The only time I could have done it was in WSW F6-7, but I bottled out of trying with wind over tide past the Holms, so I anchored off Penarth (13kn SOG next day though

)

Sounds like you don't mind anchoring, so a fair wind and tide should just about get you to Watchet/Blue Anchor Bay on a tide from Lundy. It is 30M from there to the entrance of the Avon, and you should be able to get a good SOG and arrive at Bristol for just before HW.

You can also wait out the tide in Cardiff Bay, or anchor off Penarth (dodgy on springs, though there is a deeper hole if you know where it is

). Many boats wait over the tide at Portishead, so there is no rush to get up the river before HW.