Loggerman,
There are several aspects to your question, given the circumstances you describe.
Others have taken you through the "how to do the auto", and TC rightly also points out that some helicopter ditchings might not involve an auto at all. Ditchng covers a range of events: powered flight under control, powered flight with limited/little control, autorotation with and without power, uncotrolled ditching.
What equipment you should carry is partly determined by legal requirements, but more usefully should be based on a risk assessment. A major part of that will be how often you fly over water, for what duration and how far from land, in what sea/weather state, water temperatures, night/day etc etc. It used to be the case that for private flights there were very few legal requirements. I believe that has recently changed in the UK, so that you do need to carry LJs and PLBs, although not aviation certified.
If I was planning to do a flight such as you suggest over the Bristol Channel, I would always take LJs AND wear them. I would also carry a PLB (preferably one of the GPS versions). I am also a great advocate of HUET training and would recommend anyone flying a hele over the sea does one of the courses available. The ditching stats in the North Sea show a marked improvement in survival rate once the offshore workers did their survival training.
If I was planning to fly even that short distance during the winter, personally I would want to wear a survival suit. You will only last a few minutes in the water without one. You can pick up such suits at a reasonable price. If you do not wear a survival suit, then at the very least wear clothes that will give some thermal insulation when immersed.
Any personal equipment to be used by the pilot needs to be immediately accessible to him.
If you are using a small hele, there are problems with liferafts. They are bulky and can present a hazard in the cabin. They may well be difficult to get out of the aircraft too. So long as you have set the flight up so that you will have been able to make a distress call, and have a PLB or similar, you should not expect to have to spend too long in the water. For that reason, perhaps you will conclude that a raft is not needed. If you would expect to spend some long time in the water, then a raft becomes a more useful piece of kit.
There are devices that can be used to enable evacuees to breath while they exit a helicopter underwater. Those with military experience will be familair with and recommend mini-scuba sets - called STASS or HEED. As a pilot and diver I think they are great and have my own personal bottle which I take with me. However, training for STASS is really essential. STASS is generally NOT used for offshore workers - typically they are provided with and trained to use "re-breathing systems". These are essentially a plastic bag and a tube. Once underwater, you breath into the bag which allows a limited ability to re-breath the air you have expired. It extends a person's ability to swim underwater and mitigates the effect of the initial immersion into cold water.
Extend your "flight planning" to consider what will happen after a ditching. In the Bristol Channel there are various lifeboats and the Chivenor SAR helicopter. As long as you maintain comms, the alarm should be raised rapidly and SAR resources deployed without delay and with a short arrival time. (It won't feel like a short time if you are in the water though!). A PLB will improve things too. Make sure the combination of what you provide and how you operate WILL allow the available SAR resources to have a good chance of rescuing you alive and well.
The first and most important step you have already taken. This is to start thinking about it. Because you have already done that, you will improve your chances of a successful outcome. Extend that to thinking through all apsects of the ditching. This will make everything go more smoothly, should the relatively unlikely event of a ditching occur.