T Mobile UK has the most straight forward rate to UK landlines....I believe it is 15p/minute...the same price, incidentally, also applies to calls to other mobile networks in the UK.
Virgin Mobile charges 15p/minute to call UK landlines for the first 5 minutes of use each day and then that drops to 5p/minute. Calls are timed to the second also but there is a minimum charge of 5p for each call (this may have been raised recently) But calls to places outside the UK while hardly expensive in comparison to other carriers are more expensive than what comes next. Also calls to other mobile networks are more expensive than T Mobile UK.
T Mobile UK, if calling home is part of your thing, also has a hook up with a company called Your Call....see
http://www.yourcallworld.com/payg/ which explain how to use a T Mobile UK sim card to call lots of places outside the UK for as little as 3p/minute.
It also explains how to have a T Mobile UK sim card sent to you for £1 (you can use your first hotel as the address)....
My vote in this case would be T Mobile UK by a tad...but service itself will be identical as Virgin Mobile uses T Mobile UK towers.