There are anomolies all over the place on how solar panels are assessed for BER rating. Worst aspect of this is that for most tube systems for example, there is no way of including IAM information, which would show an improvement of about 20% for tubes.
There are also difficulties in getting any solar water heating system to meet the regs for a house with a large floor area. There are assumptions made about the number of people in the house and how much water they will use. Basically they don't use enough water for solar to make up 10kwhrs / sq M.
I wouldn't design a system around maximising the BER rating unless you have to. Design it using simulators that provide you with what you need, and see if that fits the regs. If not, you have a problem...