Delbuort,
String's advice is accurate - that said my advice would be to book a Bed and Breakfast and make a weekend out of it. Bear in mind that because of tides you may be getting up very early for a dive and a 2/3/4 hour drive may not be your cup of tea.
BTW Oban is superb diving - there are walls and wrecks. The SS Breda is fantastic but so are the Rondo (my favourite) and the Thesis. One thing though, you need to have some UK dives under your belt - the conditions in Oban can get a little difficult.
Portland is a good place for you to get acquainted with UK diving - UE is an excellent dive centre and if you are a tech diver or want a DIR friendly centre that's where to go. (Try a dive on the Countess of Erne inside the harbour - it's a 14 metre dive and is often used as an intro dive to UK diving - my first UK dive!)
My advice to you would be to try and join a BSAC/SAA club and let some of the seasoned divers show you how to put up a DSMB. Irrespective of diving agency, it is the norm over here for buddy pairs to have a DSMB and be proficient in its use.
UK diving is largely wreck diving - there are plenty of wrecks over here and I'm not talking about the divers
There are also some fantastic scenic dives - Plymouth in the SouthWest is superb.
And .. you want a drysuit seriously! While air temperatures over here don't get that cold compared to other parts of the world we do get a lot of wind from the Atlantic which means that you'll chill very quickly
As for traffic - yep it can get bad - we're a small over populated island and our roads are narrower than the US. Driving over here is more aggressive than the US.