Personal liability insurance is definitely needed as it is in the range of a million.

However regarding excess insurance - how should I relate to it in the first place?
A/ It's only a few thousands, I am better off self insuring for that.
B/ Car hire companies will screw me anyways (some more, some less - are there better or worse in this regard?), GBP 500 for a scratch? So I would need this excess insurance anyways?

Is there a good comprehensive article regarding the nuts and bolts of what you should take care of when renting a car?