Wardriving in UK?

Use Google and search for wardriving.
I would also suggest spending a few weeks researching basic wireless fundamentals as your questions would seem to indicate you are way over your head.

"Just Another M.I 5 1/2 Fan" hath wroth:
It's not easy to do war driving with a wireless router. It's normally done with a wireless *CLIENT* radio.
Diversity reception. It increases the probability that one antenna will hear the signal better than the other. More specificially, it protects against "frequency selective fading" problems, where one antenna ends up in a null, where the direct and reflected signals canel. The other antenna is mostly likely not in the null and will hear the transmission.
Yes, but don't do it. There are routers that have a client mode which do work for wireless sniffing. I suspect this is not what you want.
Yep. Work on the fundamentals, learn the buzzwords, and read the various war driving web sites and blogs. the URL at the bottom of this article.

Thanks for the explanation on aerials Jeff. I had visions of all sorts of reasons for various frequencies with the twin aerials.
I have a Compaq laptop which shows an unbelievable number of unsecured routers around town but the lappy has no facility for an external aerial. It would be nice to sit indoors with a 2.4G aerial outside on a rotator which is what I was sort of asking if it would be worthwhile doing.
The price of directional aerials for 2.4G seems to be someone's idea of a joke though.

Seems to be some quite easy to construct aerials here:-

"Just Another M.I 5 1/2 Fan" hath wroth:
If you're really into antenna diversity, see: | | | The usual solutions are:
1. Add a PCMCIA or USB wi-fi card or device. You can run two at the same time, although it's good form to disable (in software) the one you're not using.
2. If your Compaq has a MiniPCI card inside, then it might be possible to snake a u-FL pigtail through a convenient vent hole to the MiniPCI card. | you do this, use the "main" connector on the MiniPCI. I also suggest you make the installation permanent as the u-FL connector will not survive more than a few insertion/removal cycles.
Sigh. Please don't try to build a radar station. The coax cable attentuation is really horrible at 2.4GHz. Unless you mount the radio on the roof next to the antenna (or better yet, on the antenna), you'll lose most of your signal in coax cable losses. This can be done but methinks there are less complex ways to do sniffing. Also, note that your wi-fi card will be transmitting as well as receiving. I don't think your neighbors are going to appreciate the resultant interference from your sniffing arrangement.
You must be looking at the wrong web pages. 24dBi big "barbeque grill" dish antennas can be purchased for about $60 US. a small panel antenna with about 8dBi gain is more than enough for casual war driving. Ask in the wardriving blogs what others are using. That should give you a clue as to what works best.

"Just Another M.I 5 1/2 Fan" hath wroth:
Yep. They are fairly easy. My favorite is the biquad and Franklin sector antennas. ignore the salad bowl. The coffee can antenna is probably the easiest to build, involving only one hole and one solder connection. Also see: easy to build reflectors.

Not Really, Go to: antenna, Orinoco Gold Card and pigtail cable are the things youneed. Please get the Orinoco gold card with the EXTERNAL input port for apigtail cable as this is very handy because soldering wires can be annoying.Or go here to view the entire store: cheap enough, you will need these. I live in Ireland and Im getting themshipped over soon. You should find yourself a LiveCD. My favourite isBackTrack, a SLAX flavour of Linux. This isnt nessecerry because you canalso use Windows, but that would be embarassing :)Use, Kismet and Airsnort/Aircrack/Airdump and Etheral. Every tool isavailable within the LiveCD [its like finding a tresure chest the first timeyou run BackTrack].WarDriving is legal, hacking into the networks without persmission isILLEGAL. Do it at your own risk but I would advise that you study more aboutWiFi Networks before jumping in the deep end [as one previous personstated]. If your not careful you could hack the wrong person and end up ingetting a beating and thrown in jail.Be careful man, dont hack unless its for a good reason. Also study/researchEVERYTHING before doing anything because you will also need to purchase aGPS device if you want to map ANY wifi spot/router around your area...unlessyou wish to study the dBi rate from the NetStumbler graphs [which I findextremley interesting and its a method that can be used also to findintruders and its very effective if you have common sense].Sorry for the very long sentences and loads of commas, its late and I amtired.-Peace out.-C Denver