Of your A, B ,C options
A, this is probably the best. You may have a small cost for property deed storage. Although lots of people get their bank/solicitor to hold them for free. You also have lost opportunity cost on potentially achieving better returns with your savings invested elsewhere (option D).
B, maximising the benefit of tax relief on the interest as an end in itself doesn't make sense. Instead simply calculate the effective interest rate you're paying after relief and compare it with leaving the money on deposit which is subject to DIRT or compare with clearing the mortgage. You'll also have the costs of mortgage protection.
C, With this option you'll have switching costs and the interest rate will be higher. You'll continue to get some TRS and the off set savings will not be subject to DIRT. You'll also have the costs of mortgage protection.