* = Armament & Displacement variation, see tables

1:600 HM s/m Undine, "U" class group 1. The lack of a deck gun and the bulbous nose housing the external torpedoes are visible here. This casing was found ( like in the "T" class ) to impair vision at periscope depth and was only found on 7 "U" class of all groups. © Andrew Arthur

1:600 HM s/m Ursula, "U" class group 1. She was the only member of the first goroup to have a deck gun, a 12pdr. AA gun. Unfortunately, there was no hatch to serve this weapon, and it was a race against time to get all crew up the conning tower and into that hatch during a crash dive! © Andrew Arthur

1:600 HM s/m Upholder, one of four "U" class group 2 submarines to retain the bulbous bow & torpedoes. The deck gun was a 12pdr. AA, but this group did have a deck-gun hatch.© Andrew Arthur

1:600 HMS s/m Una, of the main batch of "U" class group 2. The external tubes were removed and replaced with a fined off, raked casing forwards.© Andrew Arthur

1:600 "U" class group 3. The casing was fined off at the end, to allow for quieter, more efficient steaming. The gun was a 3" low-angle type. © Andrew Arthur

This class was originally designed as an unarmed "H" class replacement, for training both submarine crews and escort teams, but early in their development it was decided to arm them to make them capable of short war patrols.
As such, the first group of three units was armed with four internal and two external 21" torpedo tubes. The external tubes were in a bulbous casing like the early "T" class, and the same problems as the latter stemmed from this : at periscope depth, the casing created a large wake, and this obscured periscope view forward. In addition, Undine fitted a 12pdr AA gun in-front of the conning tower. The only problem with this was that there was no special gun hatch, and it was with a great struggle that the gun crew and ammunition got out the conning tower hatch in a rush, that the gun was supplied with ammunition with a full bridge watch, and that in a crash dive, both the birdge & gun crews and all the ammnition got back down the hatch in a hurry!
Single hulled construction was opted for, all ballast and fuel being carried internally, and a casing covered the fore ends of the full for about 3/4 of it's length, tailing off after the conning tower.
To simplify ( and compact ) machinery arrangements, they adopted diesel-electric surface drive; the diesels were permamnently connected to the generators, and for surface power, the motors were run straight from the generators. This meant no bulky, complex and hard to produce gearing & reversing arrangements were needed, and particualarly in war, gearing jigs took a long time to manufacture and get into production.
At the out-break of WW2 the three first class units were put into action, and were extremely successful in the shallow confines of the Mediterranean, Norwegian Fjords and the coastal waters of the UK.
Because of their success, they were put right back into production, and the second group differed mainly in having a permamnent deck-gun and gun hatch, and after the first four units, no bow tubes, and a fined, raked casing forwards. Also, uprated diesels and generators were fittted, and propulsion was quietened and made more efficient by adopting finer, better machined propellors. The third group had a slightly lengthened after casing for smoother surface running.
As already mentioned, they were very succesful. Being very simple to manufacture ( compared with German and Italian  submarines, they were very dated ), a real bonus in wartime and easy to train crews for ( particualarly those of the Free French, Greek, Norwegian & Netherlands navies, and the Soviet Union ). Aslo, being around in large numbers, they could absorb a disproportionate amount of the enemy's ASW resources.
It was the inshore waters of the UK & Norway and the confines of the Mediterannean that was their hunting grounds, always being fairly close to home ( never more than two days sailing from base ), their small size allowed them to work close inshore undetected, and although they were not very significant themselves, as a cxlass they were..
Being small however, conditions aboard were far from comfortable, one veteran describing a term in  naval detention barracks a holiday compared with the forends of a "U" class submarine.
After the war in Europe ended they were quickly relegated back to training, and soon paid off and scrapped, or passed onto other Navies.