I'm leaving after just shy of 19 years in the police, currently waiting in the depot driver talent pool. As previously said the wages and pension aren't bad, but not what they were (bear in mind we pay 13.3% into it each month), but it's the constant do more with less, politics, lack of support from the top and endless barriers that seem to be put up that make the job less and less appealing. I used to think it was a career in the police, now it's just a job. In my case, after making the decision to apply I had a good hard look at the options available to me - there are many different roles in the police and it can be an interesting and enjoyable way to earn a living if you find the right role. But there is a lot of competition for the 'best' jobs as lots of cops are desperate to get away from the front line and will apply for anything going.
Anyway, a year after that initial application went in and a fair bit of thinking and research into a career on the railways, I am 100% sure that if or when the offer comes I will take it. As for the worry of failing the course - well I'm fairly technically minded and can pick things up quickly, so I'm confident I'll be fine having looked into what the course involves. The TOC will be investing a lot of money in training me, so will want me to pass. If the worst was to happen and I end up without a job, I could always apply to re-join the police again as I believe that you can keep your original benefits if it's within 2 years.
I'd go for it - you've not lost anything but a bit of time at the application stage. Good luck....