A few regular breeders I'll have to confess to not having seen in Britain yet, and in some cases not at all (these really should be easy compared to some of the things I have got on my list! Just haven't got round to going for them yet and don't drive...)
Dotterel
Corncrake
Storm Petrel
Black Grouse (have seen abroad)
Ptarmigan (have seen abroad)
Parrot Crossbill or Scotbill (to be honest have seen this or Scotbill, but other than the fact they were def too large billed to be Common Crossbill I couldn't be bothered to try to figure out which...I bet a fair few people have got dodgy Scotbill on their list)
Seen all the other British breeders at least
Most common "rares" I haven't seen
RB Fly (at all! never twitched one as I want my first to be self found)
Red-throated Pipit (found one abroad as a vagrant)
Little Bunting
Buff-breasted Sand
Spotted Sand (actually found a sandpiper years ago in a flock of about 15 Autumn Common Sands - short tailed, very yellow legs, such a different call it was easy to pick up from the Commons by that alone - was too scared to call it or even ring it in, as was young and had a Ring-billed Gull rejected shortly before (which I really shouldn't have claimed as views simply weren't good enough)). Without wingbar details or any detailed description it'd never get accepted but it goes to show how the confidence can be knocked by a rejection! I didn't even know Spotted Sand had a different call from Common Sand until I looked it up after, and the call matched perfectly...confession over - as you can tell, it still hurts, and again I still want to find my own!