I am trying to determine what my father did in the armed forces during the second world war and have very little to go on apart from these three photos. The first one with three people was taken in Kalyan, India in August 1946 and that is the only info I have. I know he served in British forces, but I I don't know which branch. His name was Douglas Herbert Williams.
We know that he was in India, at Kalyan by August 1946. Unfortunately, Kalyan was a major transit camp, so this does not tell us very much. He could have been an individual sent out to join a unit in India or the Far East; he could have been moving from one unit to another; his whole unit could have been being moved from one station to another; he could have been part of a unit based there; or he could have been going home for demobilisation. So it doesn't tell us as much as it would if, eg, it was a small garrison post.
The one thing that seems to be a clue to me is the second picture. The badge on the left of the picture (his right) looks a lot like the parachute wings badge - note the distinctive 'cone' shape in the middle of the wings - which makes it likely (though not guaranteed) he was in the Army. It doesn't absolutely prove it, or tell us anything specific about his unit - various different people would have been entitled to it - but it's something.
Pending any other information, I think it is a decent bet to say he was probably in the Army, not the Navy or RAF, and make a request for his Army records on that basis.
Edit: R Leonard's answer points out that the badge on the left of his shirt seems to be an RAF aircraftman badge - a circle with one upright wing. I think the third picture confirms that they are correct, and my original guess of the Army was wrong - this definitely looks like an RAF uniform.
The hat seems to be a field service cap (see page 6 here, though I'm not quite sure why the white top not blue), and you can just see the tip of an eagle's wing on his left shoulder, which would be found on an RAF greatcoat. The only other thing I can add is that he is probably not an officer; it seems officers would have had a rank marking on the top of the shoulder, which isn't visible here.
You mention that he might have enlisted early. For someone born 1925, he could in theory have left school aged 14 in 1939. However, because civilian ID documents were common during WWII, there was less leeway for just turning up at a recruitment office and saying "oh, I'm 18, honest, sergeant" than there had been in WWI - it's possible, and it certainly happened sometimes, but absent any evidence to the contrary I would assume he went through the normal process and so his dates would be accurate in the records.
One other possibility - did your father have a distinctive name? If he was a Reginald Hebblethwaite then it might be worth searching various of the databases available on Ancestry/FindMyPast/etc, on the off-chance he turns up, but if he was a John Smith it's probably not worth the effort - far too many false positives. Service records per se are not available online, but some ancillary records are - eg the Royal Artillery personnel index cards, a few rolls from infantry regiments, etc. These are subscription services, but if you're in the UK you may be able to get access to Ancestry through your public library service.
In the second photo he also seems to sport an RAF single wing device on the left side – this the correct placement. This could be any of several what are known as brevet devices, but are correctly aircrew flying badges, for examples:
Signallers Wing (with the letter S in the center wreath), Wireless Air Gunner (with the letters WAG in the center wreath), Air Gunner (with the letters AG in the center wreath), Radio Operator (with the letters RO in the center wreath), Meteorological Observer (with letter M in the center wreath), Bomb Aimer (with the letter B in the center wreath), Parachute Jumper Instructor (with a parachute in the center wreath), Engineer (with the letter E in the center wreath), Navigator (with the letter N in the center wreath), Or just plain aircrew wing with RAF in the wreath.
I am sure there were others. I’d suggest you start your search with the RAF rather than the Army. The RAF used the same shaped parachutist badge as the Army. You may wish to peruse https://www.rafweb.org/Menu.htm#badges
Later after 3d picture As Andrew has correctly identified above this picture is assuredly showing an RAF uniform. The white top to his forage cap, I believe, is indicative of personnel under flight crew training.