UK Ltd company formation agent advice and pitfalls?

I've used a few of these services, but settled on Companies Made Simple as they seem to be the most professional. As you note, you will need the paper certificate of incorporation for opening a bank account (I believe this is included in their Bronze package, for others you sometimes have to pay extra); they offer a Barclays account as part of the package, but I would shop around (subject of a whole 'nother question).

When you sign up, you will need to give a registered office address plus the names and addresses of officers (directors) and shareholders. I would avoid using your home address as this leaves you open to fraud; the names and addresses of directors are freely available via Companies House; better to use a "service address" (which can be the same as your registered address). Note that there is now no requirement for you to have a company secretary, so you can go without there if you want.

Once you have started a limited company, you have a number of new responsibilities (as director), so I would strongly urge find yourself sound professional advice on the legal and accounting front. This doesn't need to be obscenely expensive, but failing to comply with some regulation or not understanding how to operate your business in the most tax-efficient way could cost you dearly.

One small point - these companies (and Companies Made Simple is no exception) are in the business of up-selling, so expect to get a certain amount of marketing email after your sign-up. Not a big deal, but worth being aware of.