Bobtheboater I've seen some 'cheap' SatNavs on Ebay that support Android/iPhone & reversing camera and freeview (DVB-T), 3G internet (dongle subscription required) so you can Google search in the car! Chinese imports, non branded though so they look a bit cheap, especially the menu systems.
So if you're looking for a 'bargain', I'd consider something like this off fleabay for about £250: (iPhone support, GPS, Bluetooth, Steering wheel control support, camera support)
6.2" HD Car DVD Player GPS SAT NAV Stereo Double DIN Bluetooth Touch Free Camera 6951926102443 | eBay
It looks like the most popular chinese rebranded products are 'Xtrons' and 'Eonon'. Probably best to go for that, than a total no name generic sat nav in my view, as you're more likely to find other people with the same unit on internet forums if you have problems and need to ask questions.
There are so many double din units for sale, it's quite overwhelming. Just go to ebay, and search for DOUBLE DIN SAT NAV. Then on the left of ebay menu, deselect
the popular brands, choose 'buy it now', and scroll through tens of pages of double din units at pretty good prices. At the end of the day, you get what you pay for though.
If you want something higher quality and can pay more, it seems the popular brands like Kenwood/Pioneer/Alpine don't do DVB-T/3G yet, unless you extra expensive 'add-on' boxes. Having said that if you want to blow about £900 you can get an 8'' screened Alpine INE-W928R somehow squashed into a double din if you're prepared to have a big job to fit it, and it has no DAB radio built in.
Links to the two 'bargain' in car electronics manufacturers I mentioned who do sat nav double dins with all the toys.
http://www.eonon.co.uk/
http://www.xtrons.co.uk
Probably a good compromise if you want a bargain set up, and don't need 1000's of pounds worth of ICE. (In car Entertainment).