I went to tests at Lords and Trent Bridge against India in 2011, so I'm not entirely sure of how much of what I have to say is still accurate or has changed. My experience was that the individual counties control the ticketing, with the exception of Lords which is controlled by the MCC. So there is no centralized ticketing agency.
With Lords, to get tickets to the outer you have to go into a ballot and it's a bit of a crap-shoot. I put my name down for the first four days (day 5 they sell them at the gate, and it's first come, first served) and was lucky enough to snare two at the media centre end on Day 4. That test sold out the first four days, and from what I understand I was quite lucky to get tickets. I imagine for the Ashes the demand will be even higher.
With Trent Bridge, it was a bit more like home - they showed what was available and when, and I got two tickets in the Radcliffe Road Stand on Day 3. Purchased them directly through the Nottingham County Cricket Club web page.
My advice would be to get in as early as possible - Lords holds 30,000 and is the highest capacity ground in England. Most of the other grounds are in the 15-20,000 range and demand is high. Some friends of mine who currently live in London missed out on Lords tickets this year, but were able to score some for The Oval.
Lords is absolutely amazing and up there with the best day I've ever spent at a sporting event so best of luck with scoring tickets!
