I completely understand why you'd want to do London as your first marathon - it is a tremendous experience, the crowds are great, the surroundings fantastic etc, but with the amount of runners on the course it can get very congested and decent times go out of the window. So experience, yes, completing in a good time, no - but as it'll be your first you'd get a PB anyway!
If you don't get into London, then I suggest Brighton - I've done this for the past two years and it is fantastic - the crowds are every bit as good as London, it is a decent, flat-ish course and there's enough room on the course to give all runners plenty of space. Plus there are runners of all ability, so as long as you're under the time limit (6 1/2 hours I think) then you'll get a nice medal and t-shirt. The only downside is the cost - it cost me £45.50 even with a club runners discount for next year's race, but for experience, course etc it is well worth it.