My situation was almost the same as yours, 2007 plate, 70K miles but full Lexus SH and buying from an independent dealer.
In the end I decided that I would take the money I would have spent on a warranty and put it aside to fund any repairs needed. The main reason was that I didn't want to pay Lexus prices for servicing (required for Warranty cover) and that those funds would be available for repairs that may have been deemed 'wear and tear' like suspension items or the exhaust system.
In any case, I think the warranty is only available for cars less than 10 years old, so you might not be able to get cover.
I did get a 6 month warranty from the dealer, but when I tried to claim for a leaky ball joint, they wouldn't entertain it. Other than that, and the usual brake discs and tyres, I have had no issues in the 18 months I've owned it.