Personally I prefer to take the rental companies insurance (all of these are all inclusive)
When I arrive I don't want any hassle with them telling me my insurance is not adequate. I also want want no hassle returning the car, if its their insuranse I hand it back and forget it whatever condition its in. If I had a seperate policy in UK It would involve claiming, something I don't want after a holiday.
If you can cope with the possble hassle then go for a seperate insurance and just rent a basic car if you can find a deal, not something I have ever looked for so can't help there, I just wouldn't want to do it that way myself.
Of the 3 deals I posted(if prices have not changed) it depends if you want to do the cashback, if you do then I would get the 7 seater Explorer or similar with Hertz, for the extra £23 over the cheapest you get a lot more car.
If you don't want to do cashback or want a more economical car then the USrentacar deal is probably the one to go for.