I am travellng to england. how do i compare prices on renting a mobile phone for one week?

actually, you can rent mobile phones. several companies provide short-term rental plans. but in follow-up to one suggestion i received, the cheapest way is to buy a pre-paid phone card/phone at heathrow. and i need a phone to stay in touch with my son who is ill. this is my first holiday in 5 years, the first time i have a chance to get away. but i need to be available. thanks for all your answers.
Answer
Is it even possible to rent a mobile phone? you can buy a cheap prepay one for about £30 and then put £10-20 on it which should last you the week.