I have recently gone BR and on the statement of affairs I put £50 as a total for both landline and mobile, you need to consider how much you spend on your phones and list a figure that is realistic and will be acceptable to the OR. I have not heard of anyone having to terminate their phone contract. A BR expert will hopefully be along soon to verify that
Have you considered contacting your network provider and have your mobile number transfered to PAYG. Some companies offer really good text and talk packages on PAYG for example O2 used to do 1000 free texts a month all you had to do was top up by £10 in a month (not use just top up) that gives you £10 talktime and they even offer packages for talktime aswell. Its worth looking down that route, I'm with Vodafone on PAYG and top up and use £5 a week and get free calls and texts on a weekend