On the buildings side of things - you need to ensure that damage caused by escaping water is covered, and that this covers outside pipework as well as the more usual inside stuff.
The water companies responsibility stops at the water meter/external stop-cock. If you have a leak after that it's down to you.
This has happened to us twice now

so I know that the repair can be expensive (i.e. 000s) and you also need to factor is
the loss of metered water. In each case we were looking at repair costs of about £3,000 plus the water cost.
Insurance companies try and wriggle out of the water loss element saying that the water company will let you off. That's only partly true - as it's an ace that you can only play once at your time at an address.
So, yes, I am sure you can save a few pounds by skimping on this cover - but it could bite you very badly if you are not covered.
On a positive note, the type of pipe which is susceptible to this type of failure was used in the 60s & 70s (called Alkathene IIRC) so you might be ok on an older house unless any repairs/alterations have been done. It's the black hard plastic stuff that you may have seen, it goes brittle with age and can then leak. The more modern blue stiff is fine.
Hope the the voice of experience helps here.

TTFN - Rob