In my opinion doing a year in industry is a no-brainer. It's a year's paid experience in a role similar to one you're looking to spend potentially the rest of your working life in. On top of that, it makes it easier to secure a desirable graduate job after university, because compared to peers without that experience you're highly desirable.
Different possible scenarios:
-You do a placement year and you hate it - great, you now know not to go work in that role, and have potentially saved yourself years of grief in a job you hate. On top of that, you still have paid experience working full-time in a professional role which looks fantastic on you CV.
-You do a placement and decide you love that role and want to continue with the company or a similar one upon graduation. - Great, you now will likely work straight after graduation, without having to spend months or longer competing for jobs. Although people will argue that in this scenario, you'd have been better off joining the company after graduation without a placement, because you earn a little bit more as a graduate student, I would argue that the opportunity to apply for better paid roles because of your experience, and the chance of being funded through your last year balances that out, and you'll also be likely to progress quickly ahead of the graduates who join the company without having done a placement, because of your extra experience. Plus, if you stick with the company, not only do many sponsor your last year, but many will also pay you a higher starting salary than graduates who haven't got experience with them.
-You opt for a placement year and get rejected/don't like the placements you're offered - Now you just do the normal course, but with the advantage over your appears of experience of the graduate recruitment process, and the skills gained from the placement skills sessions run by universities. You'll likely have put a strong professional CV together years before your peers, and are less likely to miss the boat when applying for positions the second time around.
The argument that you risk earning a little less money although valid, is I think minimal compared to the benefits of doing a placement year. I'd also point out that placements in the UK tend to be very competitively paid - city of London banking placements, e.g. Nomura, JP Morgan, pay around the £35k level, whilst placements in auditing firms, engineering firms tend to be around £17k-£21k. The offset between a placement year and the first year of a graduate is minimal when you consider you give yourself a strong chance of getting a better graduate job than you otherwise could.