There's some extension and compression going on but I would recommend also twisting at the top of the wave. Meaning, turn your upper body to face down the wave again to drop back in and gain new speed.

Kerry Lynn Robbert does it only happen in the water, or also when you practice on land?

Do you have any footage by any chance?

Here's a potential line suggestion that might give you more speed. Hit the far right bank in the middle, hit the bottom bank in the middle, left bank in the middle, top turn in the far right corner. (And maybe another one after that in the bottom left). It's what I do in my local oblong shaped bowl. So basically three pumps for speed and then a carve. Or you could even do multiple laps pumping and then hit the left bank in the middle and then hit the far right corner. Hope that makes some sense to you. Can share a video of my bowl if that helps but maybe the picture will be enough.

Not the UK but similar conditions. I use a hooded 5/4 in winter with 5 mm boots and 5 mm gloves. Most people over here do. I prefer chest zip over back zip. It seems warmer. Have never tried zipperless.

Well twins have a habit of sliding out if you don't surf them on rail. I'm assuming your fish would be a twin. So that could help your rail game.

A shorter board would probably also be easier to surf more vertical lines. Which also require surfing in rail.

On top of that a shorter board will give you less speed by simply cruising down the line. You would have to manufacture the speed yourself. Now of course getting it on rail is the best way to do that if the waves allow that kind of surfing. (Pumping would also work but that might put you too far out of the pocket).

Downside to a fish compared to a shortboard is that it might allow you to get away with certain things that a shortboard would not allow. But as you're coming from a 7'0 this might be an appropriate step.

Do you have access to the get the right board course? That would help you answer these kind of questions.

Hope that helps!

Ah it does only one photo and a time

Feast your eyes