In planning to swim around the UK. I really need to answer some serious questions. Particularly the best fin. I need to swim around 12 hours per day, so need a gentle fin, but one that will slow down the kicking cycle. Hard fins do that, but soft ones encourage a faster cycle. So do I choose a freedive fin or a fancy speed fin that maybe a scubie would choose. Or do I keep my cheap Russian fins that I can wear all day, without killing my muscles, but I get little thrust? Of course, I want a fin that will give me as much thrust as possible, for as little energy expended, but one that can also go fast if I need to work hard to get out of trouble. I suppose, I need to get all the manufactuerers to send me their best choice to test. Who comes best recommended?
Just returning for a moment to your original question. Fins are worn for power, manoeuvrability and endurance in the water. They rarely combine all three attributes, despite claims to the contrary by manufacturers marketing "universal" or "multipurpose" fins.
You mention "Russian fins". Do you mean the Alphaplastic Delphins illustrated below:
which are worn by finswimmers during rest periods between monofin sessions? I have a pair which not only fit my feet perfectly - the only ones I've ever tried with closed heels and toes that generate neither cramping nor blisters - but also match the gentle surface snorkelling I do for exercise and recreation off the North East coast of England. They're such a good fit and match that I almost forget I'm wearing them.
Your plan to "circumnatate" Great Britain, spending 12 hours a day doing so, suggests that your principal criterion in fin choice is endurance. You'll need an exceptionally well-fitting foot pocket and a blade that matches your strength, stamina and commitment. Comfort is paramount. There's no instant solution to that problem, because everybody's foot dimensions (breadth and height as well as length) are different. Because fins are designed to fit a range of sizes, not a single size, finding foot pockets that accommodate feet precisely is more of an odyssey than one simple trip to the diving equipment store.
I think you're right to try out all kinds of fins, open-heel scuba ones, full-foot ones and even bodyboarding ones, before you narrow down your choice to a single model. My impression from reading the messages on this forum is that freedivers normally go for a certain type of closed-heel, long-bladed fin, marketed by a small number of manufacturers. Surface snorkelling, on the other hand, can be done with many types of fin. When I snorkelled in the popular La Jolla Cove snorkelling spot a few years ago in Southern California, I observed snorkellers shod with adjustable open-heel scuba-type fins, fixed open-heel bodyboarding fins, long-bladed freediving fins and standard-bladed full-foot swim-training-type fins. All were enjoying themselves immensely, each wearing completely different kinds of fins.
So don't settle on a particular make or model for the moment. Decide what your priorities are. Buy, beg or borrow as many different kinds of fins in your size as you can muster and see what suits you best in terms of fit and performance. It's important that you make the ultimate choice, because they have to be right for you, not somebody else.