First one - on one hand a full MOT and 6 months tax on a £700 car is good, and it looks alright in the photos, but it is a cat C write off and the ad is not exactly coherent.
I would want to find out more details about the write off and all repair details. It could have been something minor, or something major. More info needed! Cat D and C cars can be good value, but they will be very difficult to sell on later. E.g. a cracked bumper is enough to write some cars off due to part and labour costs, but it could also have been in a bad crash. We don't know at this point!
ABS light being on is potentially just a sensor, but it could be a major fault. Also I
think a warning light like that will fail an MOT. Someone else should clarify that though.
Second one - no mention of MOT, service history, or Tax, and no interior photos. The 1.4 is a bit asthmatic, but for a first car it will probably be fine. Watch for electric faults on the mk4!
Third one - find out how long tax and MOT are. For some people long = 2 months. No mention of service history and no interior photos. Quite pricey for a 2003 Yaris as well, but there may be some room for negotiation there. Looks alright from the photos and they are reliable cars, so possibly worth a look.
Fourth one - Full MOT is good, but no mention of tax or service history. Also scuffs on the bumper, missing hubcaps, and filthy interior are a tad offputting. Not a lot of effort has gone into selling that car, if it close it might be worth a look but I wouldn't travel too far to see it.
Fairly average selection, but surely in London you can do a lot better
ETA - 2 mins on Gumtree turned this up:
http://www.gumtree.com/p/cars-vans-motorbikes/1998-volkswagen-polo-cl-14-new-12-months-mot-full-service-history/1015377059Even if it has no tax, for less than a grand you can have a decent looking motor with a full MOT, tax, and service history. Plenty of nice cars out there!
Cheaper still, and a very good honest advert:
http://www.gumtree.com/p/cars-vans-motorbikes/vw-volkswagon-16-polo-saloon--fsh--genuine-low-miles-59k--price-drop/1014648726