Service records for Second World War British servicemen are often the best source of information about an individual, and are held by the Ministry of Defence. They will contain information such as when that person joined and left the armed forces, and which units they served in and when.
For the Second World War period, full services records are normally only available to next of kin for 25 years after the death of a veteran. During that period the Ministry of Defence will only release only basic information to enquirers who are not the next of kin.
The National Archives hold many useful records, such as unit war diaries, and other documents such as maps, plans and reports regarding various campaigns and battles. Their collection can be searched online at their website. You can read more here about the different categories of Second World War records that they hold.
If you are researching a servicemen who died during the war, their details will be recorded on the Commonwealth War Graves Commission website which also has exhumation and concentration records available which give information about initial field burials.
Officers will be featured in the relevant Navy, Army or Air Force List, copies of which can be viewed at many local libraries. Officer’s commissions and promotions feature in the London Gazette, the archive of which can be searched online.
If you are researching a serviceman who was held as a Prisoner of War, there may be some relevant documents at The National Archives, such as questionnaires completed by ex-Prisoners of War, and also inspection reports by the Red Cross. The International Red Cross hold records of all twentieth century Prisoners of War.
Second World War gallantry medals were announced in The London Gazette, which can be searched online. The National Archives hold citations for Second World War British Army gallantry medals, and these can be downloaded via their website. The Ministry of Defence Medal Office website has information about Second World War campaign medals.
Depending on which unit a person served in, the relevant Regimental or Corps Museum, or the National Army Museum, National Museum of the Royal Navy, or RAF Museum, may have some information that may be of use.
There are a number of useful research guides available at the National Archives website.