FlyerTalk Evangelist
Join Date: Dec 2002
Location: Danville, CA, USA;
Programs: UA 1MM, WN CP, Marriott LT Plat, Hilton Gold, IC Plat
Posts: 15,722
I had a bad experience with Telestial which I will post after I receive the company's response to my complaint; however, I cannot recommend that route at this time.
I would simply buy a unlocked GSM phone from ebay (I got a 2-year old Sony model for around $70 including shipping) and then buy a Vodafone SIM card when you arrive in the UK. There are phone stores everywhere that can get you up and running in a few minutes. The Vodafone rates are far cheaper than any rates offered by US cell carriers. The downside is that you won't know your new tel. # before you leave, but you can always email it to people who need to know.
As for phone rental, try a search of this forum, there are several major companies that offer service. However, the per minute rates are very high, so you will save quite a bit by simply buying a cheap unlocked phone if you have any significant usage (> 30 min). And the next trip, you won't need to rent another phone.