If I were hiring, I'd want to know up front. Drop a bomb and they're not going to be happy.

I'd be upfront about it. In the UK, you can't be refused a job on the basis of disability if there's a reasonable adjustment the employer can make for you, and - obviously - if you're otherwise qualified to do the job and would be offered it.

This isn't a job where you need to stand to be able to do it (e.g. security guard), so whether you're seated or standing isn't going to affect your ability to do the job.

And from a completely cynical point of view, if the employer is required (either by law or by some Head Office policy) to have a certain percentage of disabled employees, it's less onerous for them to have someone with a 'mild' disability which can be easily accommodated than someone for whom they have to make much greater adjustments.

Be honest. If they don't hire you based on that, you can sue them until their fingernails turn blue.

I've seen plenty of bank tellers who are seated during the course of their jobs. I don't think it's an unreasonable accommodation at all. I would phrase it casually, "Just so you know, I had a back injury x years ago, and it's hard for me to stand all day." When you get the job, ask for a chair.

If you tell them and they don't hire you, it can be very difficult to prove it was because of the disability. If they make you a job offer, you tell them then.

Honesty is usually the best policy, but not so much in job applications. As theora55 says, tell them when they offer you the job, giving them the option to say "well, we can't really accommodate you in this position" rather than having it be a factor in your selection or otherwise. If they ask why you didn't reveal it earlier say "well, I didn't want my back issue to unfairly advantage me in this PC world, because I was worried you would feel obliged to give me the job and wanted to win it based on my skills". Or something like that.