In reply to RockShock:
Jura is probably the toughest classic fell race there is, so I would say not one to start on. But if you have the skills to do it, it is in my opinion the best. It's just an amazing place and amazing experience. If you post your entry on the day you should get a place, but it does fill up.
Most of the classic longer fell races are pre entry, you have to keep an eye on the website of the organiser and post your entry soon after they open if you want to be sure of getting a place. LIke some have said, most of the longer races need you to have experience of long races, so you might ask "If i need long race experoience to do long races, where do I start?" Do some medium races, or demonstrate you have good mountain experience on similar terrain and most race organisers will let you in.
GPS is allowed, and some people use it, most don't bother because if you are racing you don't really have time. It's quicker to use a map and compass if you know what you are doing. It is hard to find GPS tracks for the right lines so you can just follow the line on your watch. People don't tend to give away their GPS tracks as they have spent years working out the best lines and don't want everyone to follow them!
Lots of good races in my blog post here.
http://tickinghills.blogspot.co.uk/2013_12_01_archive.html
Perhaps look at Yorkshire 3 peaks, more of a trail run than a fell race, but a brilliant race with some tough climbs and will give you a taste for fell running without being so different from european running. Ennerdale is a tough race, not as tough navigation as Jura but it's long and a real classic with a bit of everything. Wasdale is also really tough, one of the classic Lake District races.
It is possible to do 2 races in a weekend, if you pick a short one and a long one, Lake District races are probably your best bet for this as they won't be too far apart.
Anything else you should know? We run races in all weather so you need to know how to look after yourself in horrid weather, get yourself out of trouble, read a map in bad visability and use a compass before your hands get too cold to use it!
Pete Bland does some race maps of the classic races,
http://www.peteblandsports.co.uk/brands/harvey/80/ really useful as they are waterproof and you don't have to flap around with a full size map in a strong wind, also have some notes as to the best lines I think.
Send me an email if you want any more advise, happy to help. Good luck!