I know you think the goal is for you to get used to cold water, but since you don't have access to cold water, the goal is not to get used to the cold water. You are not trying to get used to anything. You are going to change your definition of cold. You are going to change what feels cold to you.
There is a difference between danger and discomfort. You have to be willing to tolerate a lot of discomfort (starting right now) to change what feels cold to you.
Before we continue, I admit that I am not a scientist. What is here is based on very little research - but a lot of theory and a lot of experience. It worked for me - that is the only thing I will claim. It worked for me.
Gaining weight is not the solution. Even in the book Dover Solo, Marcia Cleveland wrote that she thought she gained too much weight for the swim. I too often hear that we should look to marine mammals as our examples on how to deal with cold water. "Look at them - they all have blubber layers." I have two primary disputes with that statement. First, we are not marine mammals. We are humans and fat is not the same thing as a blubber layer. Think about people that you know. Typically, isn't it the women turning up the heat and the men turning down the heat - even though women typically have higher body fat than men? It has been proven that muscle is a better
conductor of heat than fat. (I would point to your friend Darren as an example. From the photos I have seen, not a lot of extra fat on the boy, but lots of muscle.) Second, not all marine mammals deal with cold by having a blubber layer. Otters, the skinny little things, deal with the cold by consuming 25% of their body weight every day. If marine mammals deal with the cold with different methods, we should at least not take the "add fat" mentality as gospel.
Also, for most people that I know, the first things that go cold are their hands and feet. You have to be really packing it on to add a layer of fat to your fingers and toes.
My training is based partly on the theory that spending time in really cold air with the minimum attire will change how your body functions. You should wear as little as possible from now until you hit The Channel. If you can get away with open toed shoes, skirt (instead of pants), no coat, no hat, no gloves, you should do it. I don't even allow myself to put my hands in my pockets. During the winter, I don't turn the car heater on - I drive with my windows open. I shovel snow in shorts, flip flops and a loose shirt (a t-shirt that hugs your body is much warmer than a loose shirt). I know there are times when this attire isn't possible - for appearance or safety reasons. I know you ski, so of course be safe. I have to wear a suit and tie to some meetings. Etc. But the point is, that you never wear something warmer just because of temporary discomfort.
There is also the theory that there are two kinds of fat - white and brown. White is the normal, big, bulky stuff that we typically think of. Brown fat is a special layer that protects our vital organs. (Again, I am not a scientist and it isn't my theory, but I believe it.) You cannot build up brown fat by over-eating. The only way to build brown fat layers is to be cold. You have heard, I would think, about people saying their blood thinned in warm climates. What you want to do is the opposite - but you aren't thickening your blood. You are thickening your brown fat layers.
Take every opportunity to be outside, underdressed, in as cold of air as you can take. Right around the first of the year it was 28 degrees here. I took the garbage out wearing only shorts, flip flops and a loose shirt. My neighbors were walking by at the same time and we ending up having an hour long conversation. I never got cold.
Because I believe that muscle is a better conductor of heat than muscle, I have spent quite a bit of time doing exercises to strengthen my stomach muscles to help keep my core warm.
Also, sit in cold water in your bathtub. Don't start with mild water and think you have time to work down to colder water by next July. Start with water at 60 or colder - and put just your feet in it for at least 10 minutes. Work your way up to at least half an hour (depending on how big the tub is, after a while your body heat starts to warm the water). Read the paper. Do work. Facebook. Whatever. After you can handle your feet, trying kneeling down. Then a few weeks later, try sitting. Then your whole body. At the end, you should be able to do an hour - all in.
On the day of the swim, be an otter: Take in lots of calories. And if you ever feel even a little cold, you aren't swimming hard enough. Swim harder and take in more calories. (I have a little rule with myself that I never decide if the water is cold or not until I have swam 15 minutes - that gives me a chance to build up a sweat.) Also, right before your swim, you should be completely bundled up to the point that you are uncomfortably warm. Hitting the cool water will be a relief. Because you are on a relay, you will want to bundle up again as soon as you can after each leg - get overheated on the boat. The water will feel great.
None of this is extremely complicated or sophisticated. However, it does take daily determination. The water was 58 degrees the first half of my EC
swim. Everyone on the boat was wearing coats, hats and gloves. I never got cold. Not even for a second.