I was in a similar position to you over 12 years ago, I was working as a firefighter over in Germany but my experience counted very little. The only time it helped me was when I needed a reference as part of the application process and at my interview. At the time, I couldn't understand why, but with hindsight, I understand it now. There are lots of different proceedures that the UK fire services do, especially when it comes to wearing of breathing apparatus. The pumps may work on the same method inside the pump housing, but the training for pump operators is very different and these skills are vitally important to get right. There are some very specific things that we do when we put up ladders and generally, we use many more different ladders than US firefighters. You would not be able to be an effective member of the ladder crew without this training.
All in all, I know that completing the full UK firefighter basic training and probationary period, made me a better firefighter and it put me on an equal footing as my colleagues. The knowledge and skills that I have from my time as a German firefighter are still with me and I have used them on several occasions to my advantage. I feel that if I hadn't completed the full training, I may have done things differently to my colleagues which could have lead to confusion at incidents and lets face it, a fireground is no place to find out that you're doing something wrong and don't know how to do it right.
Good luck with your application to join the fire service in the UK and if you need any advice, feel free to contact me using the email link next to my name.
Kaiser