That would depend on just what is meant by "copied another journalist's work substantially". One may not, under UK copyright law (or indeed under any copyright law) simply copy the words and phrases used by someone else, even with trivial changes, to avoid the effort of composing one's own words to express an idea or describe and event. a close paraphrase will generally be considered an infringement of copyright.
However, copyright does not protect ideas. A journalist may describe an event, basing the account on the3 accounts of other journalists. As long as the new account does not directly copy or closely imitate the source accounts, it will probably not be treated as an infringement of copyright.
The "fair dealing" in UK law is a narrower exception to copyright than the US concept of "fair use". According to the Wikipedia article:
Under the Copyright, Designs and Patents Act 1988 (CDPA), fair dealing is limited to the following purposes: research and private study (both must be non-commercial), criticism / review / quotation, and news reporting (sections 29, 30, 178); as well as parody, caricature and pastiche (section 30A) and illustration for teaching.
So unless the use by the second journalist fits one of those categories, fair dealing will not apply. If the copied content is incorporated into a commercially published article, it fairly obviously is not for the purpose of "research and private study" or "illustration for teaching". It might well, however, be for "news reporting" or one of the other fair dealing purposes, and the detailed facts of the matter would be used to determine if this was a case of fair dealing or not.
Article 2 paragraph (8) of the Berne Copyright Convention says that:
The protection of this Convention shall not apply to news of the day or to miscellaneous facts having the character of mere items of press information.
Section 30 paragraph (2) the UK Copyright, Designs and Patents Act 1988 says:
Fair dealing with a work (other than a photograph) for the purpose of reporting current events does not infringe any copyright in the work provided that (subject to subsection (3)) it is accompanied by a sufficient acknowledgement.
Taken literally these provisions would seem to mean that there is no copyright at all on news reports in the UK. I do not believe that that is a correct interpretation. I think that those refer to not attempting to prevent the use of copyright from precluding the reporting of actual events, where a previous report is a matter of "bare facts". I don't have any case law on how Section 39 (2) has been applied.