Carlisle has been attempted a couple of times; Autair did so in the late 1960s as well, which also like Euroair's later effort, extended to Dundee; a sort of chaining all the least likely points together.
There's some business traffic on offer to the Cumbrian industrial coast (Workington/Whitehaven), which are otherwise an impossibility to get to from London in a sensible time by any means. Went with a colleague to Whitehaven and he asked me what was the closest airport to there; was not too amused when I said Ronaldsway, because he actually looked it up
I believe that Scot Airways some years ago looked at a Do328 service from London City to Carlisle; better aircraft (smaller, cheaper, faster) and a better destination than one down at the wrong end of Essex. If they felt it was not worthwhile I wonder what has changed.