Usually, the extra $5 an hour goes into the FBO's pocket and they give vague promises that they will not come after you for the deductible. So no real protection there at all, and the insurance company may decide to come after you anyway so you have wasted $5.
Renter's insurance provides a more reassuring coverage of your deductible, and you also get some liability insurance thrown in. I have enough liability insurance in the States to keep any plaintiff happy and stop them coming after my house. It's purely a guess on how much that should be. Covering yourself for tens of millions is just too expensive, I assume most people will be happy with a fraction of that.
Make sure you understand subrogation.
You need to make sure you are covered for medical too, most UK off the shelf travel insurance will not cover you for light aircraft, but you can get coverage for extra premium.
Don't think you can return to the UK and get away with it, you cannot.