There are no useful alternatives to using Athens airport. See here for a map of Greek airports:
http://www.mapsofworld.com/international-airpo rts/europe/greece.html
(Neither Larisa nor Volos receives flights from the UK).
However, I'm confused by your statement that 'the only flights to Athens are from Gatwick'. Luton Airport is only 84 miles by road from Birmingham Airport. (Alternatively, take a train to Milton Keynes Central and pick up the coach link to Luton Airport from there- you can buy a through ticket).
easyJet operate from Luton to Athens:
http://www.easyjet.com/en/book/index.asp
Manchester Airport is 90 miles away from Birmingham Airport, by road. (Rail travel is possible by changing at New Street and then at Crewe, Stockport or Manchester Piccadilly, as appropriate).
Olympic Airlines operate from Manchester to Athens:
http://www.olympicairlines.com/
Either of those options would be far easier than travelling long distances across Greece.
Chris