This looks like a fantastic idea. I've not tried it yet, but they certainly have a lot of endorsements.
If it's good enough for Steve Krug, it's good enough for me.
Set up the test you want (what site or sites, the scenario, the tasks) and select the number of participants you need. Then sit back and wait for the results to come in. Every participant produces a video of them completing your test, plus a write up.
At $29 per participant, it would be pretty hard to do the work yourself at this price. Particularly if you want to involve external participants from specific demographics, and you're going to have to pay them.
Although this is a US-based company, you can select UK-only participants too.
I'm definitely going to give it a try in the near future.
usertesting.com - fast, low cost usability testing
Follow up: I did some testing a month or so later and was very happy with the service - usertesting.com big thumbs up