Back in those days British prisoners were not allowed to wear their own clothes. whereas today they can wear approved clothing of their own - if they are at a level of behaviour privilege that qualifies it - Its part of the Incentives and Earned Privileges scheme (IEP)
So - back to the question, UK prisoners had to wear long sleeved shirt rather like this, also there was a lot of former military clothing stock around which they found a use in prisons and in nationalised industries such as railways and docks
http://www.selftrading.co.uk/HM-Prison-service-made-shirts-vintage-clothing-clearance-290708
These were issued in either blue stripe for convicted prisoners or brown stripe for unconvicted (remanded) prisoners
They also had to wear prison jeans.
We used to weave all the materials into whole cloth, and then make it up into clothing - I remember all the weaving, winding and cutters workshops - we buy all that material in these days and then make it up into clothing.
Those jackets in the link by eburacum45 later on became something of a status symbol - their issue was discontinued but prisoners who had had them issued we allowed to retain them. As time passed then the only ones who had them were the very long term prisoners - which eventually resulted in them being owned by ‘lifers’ - mostly murderers and serious organised criminals. The idea being that if you had one of these jackets then you were a ‘somebody’ in prison because of the severity of your offending. Naturally these became desirable and were traded between prisoners - I have not seen one of those lifer jackets in years.
In the second link posted by Eboracum45 that clothing is in far too good condition - its usually tired looking and well worn - prisoners do not own prison clothing so it gets all sent collectively to the laundry and they get whatever is available issued at kit issue time - it tends to fit only where it touches
Here is some typical prison issue clothing
The green pants are issued to prisoners who have jobs where they are likely to become dirty such as building trades training, painting work parties, grounds maintenance parties etc. Their footwear is rubberised plimsolls, never shoes they might be issued with protective boots with synthetic shell toe protectors, those boots are always brown and always slip on - no laces.
Prisoners who behave to a specific level can wear their own clothing if it conforms to certain requirements - so no gang related logos, racist logos etc, no office type shirts or any clothing that bears even a slight resemblance to staff uniform - they tend to wear designer labelled sport type stuff - which is frequently snide - wearing designer label stuff is a status thing in prisons but it can also make the wearer a target for robbery.
There is a whole layer of prison politics in prison clothing, those who are trying to prove their status will try to express it in clothing, whilst you’ll get some prisoners who wear all prison issue stuff because they feel they don’t have anything to prove to anyone - these are often the real serious prisoners and its something of a warning sign to other prisoners not to mess with them.
Prisoners who wear ‘designer’ clothing are often trying to prove that they are players who can make money dealing drugs or other contraband, they are trying to portray themselves as ‘Big Misters’ The clothing demonstrates their wealth and power - so it makes it easy to target them for drug and mobile phone searches so we can bust them down for dealing - maybe if we get lucky we get enough evidence to get them in front of the courts and have another jail term added, and they also lose their personal clothing privilege too
Of course there are other prisoners who have almost nothing of their own and have to wear prison issue clothing because they simply cannot afford to buy their own or have it sent in by relatives.
Prison clothing, like prison politics is all about context - who is wearing what and why, but if you have been in this game for as long as I have, then you can pretty much pick out the meaning at first glance - which is useful because it can help you understand pretty quickly what sort of person you are dealing with and ways you might use to handle various situations.
Its subtle things like that that can made a big difference, it takes years of experience and unfortunately due to cuts on staff numbers and a crap staff grading system - they have a massive staff turnover and the experience has simply walked away to other jobs - so there is much more unrest and violence in our prisons.