There are thousands of contract options but the cheapest pay-as-you-go options are Lyca, Vectone and Lebara (maybe Asda too) - these will let you call Canada for 5p or less per minute. You can buy a sim card in many shops for 99p or so - you can buy top up vouchers over the counter as well. If you don't have an unlocked phone, you can get one at CarPhoneWarehouse.
Broadband options will depend on where you live and whether you have a landline but the green tab at the top should help. Some providers will let you sign up for a month at a time. With others, if you sign up online or by phone, you get a week in which you can judge the service and cancel without penalty if you are not satisfied. If you don't plan on getting a landline, you can look at mobile internet although this may not be suitable for high usage. There are some packages which provide broadband, TV and phone services together.