In reply to timmy a: My prefered forum is UK Divers
http://ukdivers.com/community/ but as someone else said probably the busiest UK dive forum is Yorkshire Divers.
UK diving is very different to the clear waters you get in the Med or Red Sea and as such is much more challenging so you are right to limit your depth whilst you get used to the unfamiliar conditions. You also need to be confident with dSMB deployment as most boat operators will expect you to ascend on a bag.
Portland I have dived with Fathom and Blues and also been recommended Scimitar. Both will take bookings from buddy pairs rather than requiring you to fill the whole boat, which is great if you are diving as a buddy pair rather than a club.
I personally would recommend the Countess of Earne as a dive as she is well preserved and shallow, the only downside is that like most of the dives within the harbour you need good bouyancy to avoid kicking up the silt.