I think you are on fairly safe ground in concluding that these newspaper contents are now in the public domain. The law at the time said that the copyright term was the lifetime of the author plus 50 years, and this did not change until 1995 and the 20 year extension only affected works which were then still in copyright. So provided the authors/journalists concerned died before 1945, their work would not have been affected by the extension of 20 years. For any copyright to remain today, the last such author would have need to have lived beyond 1949. This is highly unlikely for the following reasoning: the average life expectancy in the mid twentieth cenurty was 75-78 years of age; if a journalist had lived to that age and not died until, say, 1950, he would have been born between 1872-1875, meaning he would have been no older than 17 at the end of the decade you are interested in, and so highly unlikey to be a named journalist authoring pieces for a national newspaper such as the Times or Telegraph.
In the event that a piece you are interested in had no byline (or the byline was something like 'a staff reporter') then slightly different rules apply. They can be found in Section 12(3) of the Copyright, Designs and Patents Act 1988 which says as follows:
(3) If the work is of unknown authorship, copyright expires—
(a) at the end of the period of 70 years from the end of the calendar year in which the work was made, or
(b) if during that period the work is made available to the public, at the end of the period of 70 years from the end of the calendar year in which it is first so made available,
subject as follows.
(4) Subsection (2) applies if the identity of the author becomes known before the end of the period specified in paragraph (a) or (b) of subsection (3).
In other words these sort of pieces are categorically in the public domain.
A word of warning about your sources. If you use microfilm (for example the microfilm from any public library) as your source you will be on safe ground. However although in theory the same rules on copyright as outlined above apply to newspapers which have been digitized on the British Newspapers Archive
(BNA) website, the terms and conditions of using that site (and the main FindMyPast site) prohibit commercial exploitation of their resources without permission. They also make the following statement which is untrue: "All newspaper images on The British Newspaper Archive are under copyright and require the right holder’s permission to reproduce in any non-personal productions." For the reasons given above the copyright in the older original works (ie the paper versions) is no longer extant, and this confirmed by a statement on the BL webiste
Most of the British Newspaper Archive features out-of-copyright material pre-dating 1900;
Furthermore as the images which appear on the BNA website have been produced by entirely an automated process akin to photocopying, they are not subject to copyright as 'new' images because they lack any human creative input. (see page 3, second paragraph of this Copyright Notice
produced by the UK Intellectual Property Office).
Given the poor quality of automated transcription on the BNA, you would probably prefer to use the microfilm where ever possible. I think I am right in saying that the British Library itself no longer holds any microfilm of backcopies of newspapers, and that you would need to visit the British Newspaper Library reading room at Boston Spa to view the actual hardcopies. All that is available at the BL St Pancras site is free access to the BNA digitized images via computer terminals.