The laws around found photos are laws of copyright. You own the print, you may own the negative, you do not own the copyright, which may or may not have expired.
In many cases the copyright of photographs like these in the UK actually has expired, due to some weird quirks of changing copyright laws; it's not as simple as life plus XX years. In fact, for some period of time it is "date of capture plus fifty years".
See the link below, which caveats that photos taken for publications (work for hire) in this sort of window of time are likely still in copyright to the publication.
https://www.londonfreelance.org/feesguide/index.php?§ion=Photography&subsect=Copyright&subsubs=Copyright+in+pre-1989+photographs&page=Advice
So there's a good chance that the photograph above would be in the public domain already, were it not for the fact that it looks rather professional and is probably in the archive of some publication. Though again the article above suggests that there may be overriding precedents from earlier Acts.
Good links on that page to details of earlier Acts.
Personally I am with Photowallah on colourisation. I think it doesn't add much, and in some situations (for example, colourising photographs of non-white people) it can distort history in a way that is complex and troublesome.
Edited by Unfocussed Mike