Hi Ian
Having read all of the reply's to your post, and reasonably experienced in navigating at sea, at the same time recognising the limits of inland waterways boat on the open sea I will make these observations.
1. Generally the Bristol Channel is not a place for the inexperienced or faint hearted in a small boat, but having said that we all have to start somewhere.
2. On Saturday 26th July I was off the south Wales coast in the Bristol Channel, the conditions were the perfect millpond, the wind strength was Zero, the wave height was zero, you could have taken a rowing boat from Porthcawl to Minehead with no problem.
The point being is this, if you want to wait (probably for a long time) for the correct conditions, and have enough experience and / or a pilot to take you up the Somerset coast and into the Avon, then in a well found small vessel it is possible to undertake the journey by water.
Certainly if you can get up to Bristol you have two choices, either enter the Kennet and Avon Canal and proceed by water, or there is a boat hoist there that will lift you onto a lorry at a cost probably much less than using a mobile crane anywhere else. I could probably dig out the name of a haulier or two who could do the job.
There may be a boat hoist big enough in either Swansea or Penarth marina, I'm not sure, but it might be worth checking out.
Hope this helps.
Paul Hayes