There will be species that, no matter how many years you have been looking, still evade you. These are mine:
For all of the hours spent seawatching I have not connected with either Cory's or Great Shearwater. The more pragmatic amongst you might point out that SE Kent is not the best place to see either of these species, and I'd have to agree with you. My seawatching time in Cornwall has been limited - a concerted late summer blitz to that fair county should supply me with them if I so desired. Corncrake for the 'southern' birder is unlikely, unless you are the holder of a 'golden ticket' and manage to flush one of these skulkers on passage. I've not been to the breeding stronghold of the Western Isles, although the introduction programme might just up the chances of a chance encounter. The same might be said of Capercaillie - I'm not going to bump into one of these on Leith Hill (Surrey) and if I did I would expect to have my optics confiscated at once. As for Scottish Crossbill, I will ask only this - do they actually exist?
I have been a little lazy in seeking those 'local' species - I could hoover up Heath Fritillary, Lulworth Skipper and Glanville Fritillary was a small amount of effort. maybe 2014 will see me do so.
Just dealing in 'macros', the closest species that evade me are the Foresters (Common, Scarce and Cistus) and Wood Tiger. I know the sites, I've even been to some of them, but my timing has been all to cock.
Leaving the colonising species alone, the one stand out omission for me is Club-tailed Dragonfly. Present nearby in the Thames and Arun basins, this should be easy enough if I put in a bit of time.
There are plenty more, and one of the reasons that spur us on to keep on looking and searching. But it's not all about the lifer, is it.