Hi
Wow you have big plans ! Hope you have a great time !! Personally I would turn your trip upside down and start in the uk and work down but it sounds like you already have quite a few set plans that you need to work with, I have a few suggestions that may help:
Why not fly from Athens too Rome? www.easyjet.com have deals around euro 30, that way your not starting your eurail time with a lengthy journey,that from friends experiences isn't much fun, then make your way to Barcelona from Rome taking in some of the italian citys, on the way ,I would forget about venice though, as it would be a bit of a detour, an alternate to the train here may also be able to take a flight from Milan (Bergamo) to Barcelona with www.ryanair.com (El Prat is the major airport the others are regionals) . From Barcelona I would head to Madrid then down to Seville, from Seville I would probably fly to either Brussels or Amsterdam (again Ryanair do a service to charlesroi a regional airport for Brussels ) or try www.iberia.es for the major airports. I would take the first three to five weeks to do this leg of your tour, this is where I would stop running with the shedule as you will be right in the heart of europe , especially if you choose Brussels. This will allow you to do at least some of the places on your wish list the longest you would have to travel is 9 hours to Prague (this is ruling slovenia out) .
Going back to the shedule tho I would book your point of entry to the uk and also plan how your going to get around as your rail pass won't cover you here, also if your planning on travelling from London to Scotland I wouldn't rule out www.ba.com and www.flybmi.com as they fly from london heathrow and the increase in fare from the budget airlines can sometimes outweigh the costs of getting to the regional airports, having to buy refreshments etc...Also unless you book well in advance air fares are usually cheaper than train.
Budget once you've paid all your travelling expenses divide your remaining funds by the amount of days of your trip, I would probably then take off about 20 euros a day for when your in the cheaper countries Greece, Italy,Spain any eastern bloc countries you visit that way you'll have extra money for the more expensive countries and I wouldn't count on the uk being cheaper because you won't have to pay accom costs , its quite expensive here and plus it will be Christmas and New Year so you'll probably want a bit extra money .
Finally enjoy have fun, don't stick to a shedule too much if you arrive in a city you've dreamed of visiting for years to think actually I hate this place , you have your eurail pass just leave you never know what you might find in a place you've never heard of and only go to because its an hour away or because it had a funny name. When diaster strikes, don't panic deal with it, nothing can't be solved, if you do visit Seville the locals will think your crazy for visiting so late in the year and tell you to come back for the feria's or when its warm,
have a great time and hope this helps !!