Car Audio Security or HiwayHifi
I've recently bought an MP3 head unit from CAS and they were the cheapest on the net.
When I went to the shop I told them I'd been looking at their website what the head unit I wanted was, and they sold it to me for their internet price (£20 less).
Have a look at their website, and they also have a forum for questions.
(in Nickgee's posting)
If you are going to go their shop, it is very close to Tottenham Court Road station (on TCR), but their shop is at the back of another, so remember the address.
They do fitting, and sometimes do this at the weekend (inc Sundays) so you can avoid the congestion charge.
Or you could try HiwayHifi, who are also reasonably cheap, and are located close to Edgware Road Station, so just outside the CC zone.
I think they also do fitting.
These guys would only do me a partial discount on the shop price after I told them the price I had seen it on their website for, but got it there and then, and still cheaper than off the shelf.
http://www.hiwayhifi.co.uk
I've only ever bought kit from these shops, so I can't verify the quality of the fitting work, or the cost, but would expect them to be reasonably decent.
Good luck and I'm sure they would be happy to talk to you on the phone about your requirements.
Cheers
Sparky