As I deal with motor insurers every day at the solicitors firm I work for, you can get Third Party cover only but I don't think it's very common from what I've noticed.
You are right I believe, Metric, that TPFT cover will not build up your 'no claims bonus' but that is because you aren't technically making a claim against your insurance (ie: you pay for any damage to you own vehicle at all times---unless the other party is at fault in which case their insurers would pay for your damage). But I could be wrong as other Parties can make claims against your policy thus losing your 'no claims bonus'. I don't work in insurance so I don't know for sure.
This is the coverage we have as our vehicle is old enough (92 Nissan 100nx) that it isn't that important what it looks like to us anymore and any costly repair wouldn't be worth the vehicle value so it would probably write it off.
I would say that comprehensive insurance would be important if you have a new-ish car (keeping in mind that cars here depreciate in value amazingly quick, imo) or if you are vain about your car appearance....or rich!
There's arguments for each policy. It just depends on your priorities and situation.
There are competative "cheap" insurers over here just as there are in the states and the rates I think vary as much as in the states too. Back home, I had what would be considered criminally low insurance rates for the area (driving 17+ yrs, over 30 yrs old with no motor convictions) but it was still more expensive than the rate we're paying here (husband only with less than one year license).
Do some web searching for free online quotes and see what you come up with!
Gah!! Work related!! I'm stopping now!