In addition to the earlier comments about validity, there are a couple of other issues.
0) Why buy your ticket starting in YYZ? Canada is an expensive starting point. If you are going to be in London from August until February, during this period, why not pop over to ARN or GIB and start from there. How much you save will depend on the class you plan to fly. In any case, it is likely to more than cover the initial YYZ-LHR segment. Thus you would gain an extra segment - and perhaps more importantly give you an extra 6 months of travel.
1) Currently there is no OW service NRT-BKK but that is one you would be able to add when JAL joins- which should be well before you plan to fly.
2) When you say that the locations are “locked in”, note that they can be changed drastically if you are willing to pay the reissue fee. Many of us change our tickets several times during our travels.