I dont like you poll choices either!
British sub campaign would be very nice and very doable.
Not famous enough to justify a stand alone sim - but perfect for an SHV addon
1) Playable boats would be S, T and U classes. based on numbers constructed.
S-class (66 built) Comapariable to a Type VII
T-class (58 built) Comapariable to an Type IX
U-class (44 built) Comapariable to - a cross between a Type VII and a Type II
2) Full interiors may have to be somewhat generic / improvised though, as there is little in the way of avaliable material for research (very few photographs and no exsiting boats today)
3) Theaters of operation,
North Sea against the Germans & occupied Norway, (1939 - 1944)
Med against the Italians & German occupied north Africa, (1940 - 1944)
Southeast Pacific. Against the Empire of Japan. (1943 - 1945)
(In that order)
4) Ports & bases include:
North sea bases: Lerwick, Dundee, Portsmouth, Blyth, Rothesay and Holy Loch
Med bases: Malta, Gibralta, Algiers, Alexandria, Port Said and Beirut
Far east bases: Trincomalee and Fremantle
5) Would need a decent aray of historical Italian and Japanese NPC Units -the latter could be ported from SH4
Would require and increased amount of Axis traffic but thats about it

LINKY
http://home.cogeco.ca/~gchalcraft/sm/index.htm