If I remember rightly (I last looked at seat 61 about 12 or more years ago) London to Athens can be done in a little less than 48 hrs. London to Paris (Gare du Nord), short trip across town to get the overnight sleeper to (I think) Bari for the ferry to Patra and, back then, it was a coach ride to Piraeus as the rail line was being modernised and I have no idea if that project was ever finished. The last mainland stretch can't be too bad as Patra to Piraeus is a decent road and by car I used to do the run in under 2 hours.
I had some questions about the route and accommodation on the sleeper train (I was still running my web site for travel UK/Crete) and I e-mailed 'the man in seat 61'. Not only did he answer all the queries but gave his opinions about dining facilities/quality of food/service, comfort of bunks etc., and was generally very informative.
I didn't do the journey personally and am unlikely to do so in future as I've no great desire to visit the UK again but I have been thinking about other trans-Europe train jaunts. No problems about taking liquids, including alcohol, on the train; no great queues for security; windows with views of more than clouds and the occasional mountain top (and no chance of running into one); decent meals with real cutlery, no excess baggage worries; with a bit of planning plan some stops for a couple of days at interesting places etc. What's not to like?