I just did this recently. My host sent me down to the local Car Phone Warehouse with my phone in hand. For 10 pounds, I bought the least expensive Mobile World SIM card. The person at the desk showed me how to insert the card, and supplied me with a price sheet. As I recall, phone calls outgoing within the UK cost me 15p a minute. Incoming calls were free. Do you live in the US? Because, believe it or not, outgoing calls to the US cost me only 5p a minute!! What a deal!!

You ask "How much time is typically on the three to fiver cards?"
Well, I didn't find cards that cheap - five pounds was the least expensive TOP-UP amount. About the time - it depends on where you're calling. If you stick to UK calls exclusively, you'll get 66 minutes of talk.

And, "which company is most reliable?" With a Pay-as-you-go SIM card, they're all the same. If you shop around, ask for the price sheet, to see how much they're charging per minute of call.