What you can do, if anything, depends on circumstances.
When claiming payment from an infringer it's advisable to think through the possible outcomes when considering how much to charge.
It is not the best idea to approach the average High St. solicitor with a copyright dispute, since it is a specialist area of law.
Google is your friend. If you are dealing with infringement by a print publication you will likely be able to locate the publisher's registered office easily enough.
The answer to this ought to be straightforward, the publisher is the party who commits the infringement.
Retrospective licenses may be offered as a way to legitimise unauthorised use after the fact.
It can be. Copyright law in UK allows for criminal prosecution where there is deliberate intent or passing-off in a commercial context.
The first thing to do is to get evidence of the infringement. If your work has been used without permission, get a copy of the publication.