I'm not a legal expert, but have unfortunately had to both make lots of people people redundant, and been made redundant (twice) myself.
If you are at the stage of being notified that you are "at risk" of redundancy and are in a pool for selection, then there's not much anyone can do for you at this point - as you've not been selected for redundancy, so you have nothing to challenge.
If you have already been notified that you have been selected and you are believe your employer has not acted properly in either the process, the selection criteria, your scoring, or your benefits, then either a union rep if you have one, or if not an employment specialist solicitor.
I've seen too many people rely on info from "friends" and unofficial/amateur "experts", when they challenge their selection to their cost, that I'd say that if you genuinely feel you have an issue (but read carefully everything your employer has given you, and your employee handbook/ T's and C's), then spend a little on professional advice - but they (like the Citizens Advice Bureau) will need copies of those docs and that info, before they'll be able to tell you whether they think you have a case or not.
If you work for a decent company, they will have done their due diligence and followed the processes and you'd be unlikely to prove they're wrong (they should have a HR advisor who has guided them through every step of the process). If you work for a company that don't follow the rules in all other HR respects and generally treat their workforce without the right level of respect, then I'd go for their throats with an employment solicitor (as they'll likely cut corners in their process).