As the websites you designed were done in the course of your emplyment, section 11(2)
of the Copyright Designs and Patents Act (CDPA) 1988 says that the employer is the first owner of the copyright in the work that you created. The fact that the agency has now ceased to exist doesn't mean that copyright has also disappeared; it will either form part of the assets of the company or it may have been passed on to the owners of the respective websites as part of the terms of each commission.
A lot depends on the extent to which you want to showcase each site. If you just wanted to include, for example, a screenshot of the home page, then that would probably be acceptable using the quotation exception in section 30(1ZA)
of the CDPA. You would need to credit the agency as the owner of the copyright. Alternatively, if you wish to demonstrate more details such as, for instance, some interactive features or Flash or scripting, you might be best off just linking to the actual site in each case, provided that the features are not behind a login or paywall. This is perfectly legal without getting permission from the siteowners, as technically speaking you are not copying the site or its code etc onto your own server, and are merely allowing a visitor to your site to access the relevant content directly from the server hosting the particular site. Even if the content is framed on your site, this still remains non-infringing - see the judgment of the Court of Justice of the European Union in a case know as Bestwater
However if you want to display more than, say, a single page of each website and the linking option isn't appropriate for your needs, you will need to get formal permission to showcase the work as part of your portfolio. The fact that you were the sole author of the websites makes things simpler as you don't also need to seek permission from any co-authors. I suggest that you try to locate the previous owner or chief executive of the agency you worked for and find out if the assets of the company (eg the goodwill or the client list) were sold on to another entity from whom you can get permission. If that proves impossible, go directly to the owners of the websites concerned as they will probably hold exclusive licences for the copyright. You never know, by making contact you may get these website owners as clients for your new business.
I hope this answers you question.