Web consultancy and marketting without previous projects

We have recently set up a small company (2 people), aiming at writing tailored web application for small and medium businesses. Since this is a new company, we have no projects we could show to potential customers.

However, both of us have a wide experience working on huge projects for huge and well known companies in UK. All those projects were managed by our previous employers. Is it ok to put those information (including those huge companies's names) on our business' website?

In general, I'd say yes. It's okay. You're effectively posting a copy of your résumé on the site, and any paper copy would include those business names. That said, I'd be cautious not to give away too much. It widely depends on your employment contracts you had with these big companies, but in general I would shy away from super specific details on what you did there, particularly unless someone asks.

If you're still in doubt, though, you could also link to your LinkedIn accounts. Virtually nobody will ever mind you having your work history (including business names) there.

Since it sounds now like LinkedIn might not be acceptable, if we're talking about big companies I would just post it on your site. Like, say, Microsoft wouldn't mind a former employee saying "I worked at Microsoft for ten years." But if we're talking smaller businesses for whom you've consulted, you could always reach out to them. Presumably you left on good terms if you're looking to advertise your time working with them, so just ask "hey, is it alright if I list you as a company I've done work for in the past?" and they'll probably say yes. It's free advertising for them, too.