Melody,
An International Driver's PERMIT is not only a good idea, it's compulsory in some countries (Italy, Greece). This is not a "license" but rather a Permit that provides a translation of the terms of your home D.L. in various languages. It must be used in conjunction with your home D.L. and NOT by itself. In Italy, failure to produce an IDP if requested can result in fines ON THE SPOT! Failure to pay the fines will result in the vehicle being impounded (and that includes rentals).
While I don't personally know anyone who's suffered that fate, it's very possible (especially now that cash-strapped governments are looking for any source of revenue they can find!). For example, the government here has just announced a "windfall" from increased driving penalties (I believe over 20K tickets issued and about $30M. collected).
IDP's are VERY easy to obtain, and it's really a good idea to have one "just in case". These can be obtained at any AAA office for a small fee. Two Passport-style photos are required, and some AAA offices have provision to take the photos on the premises. The IDP is valid for one year.
For estimated driving times, have a look at Map Quest. Whether the times are "realistic" or "ballpark" is a matter of opinion. I most definietly agree with your friend in Wales regarding the times. I've found that espeially for driving in the U.K., it always seems to take longer than estimated. There are only limited Motorways (Freeways), so some of the roads are narrow, winding two lane "country roads". Keep in mind that you'll also be driving on the "correct side" of the road, so will probably be operating a bit more cautiously than you're used to.
Cheers!