When to buy . . . last week when the sale was on. Other than that, there is no better or worse time to buy.
In practical terms, I would buy the pass now and then wait. Other than the “must start using it within 11 months of buying it” rule, there’s no good or bad impact to buying closer to March. If you buy it in February and then find you can’t travel in March for some reason you have until January 2028 to use it. If you buy now and your plans change you have until September 2027. If you travel in March as planned it’s all moot.
Eurostar - pass reservation required. TGV - pass reservation required. LNER - no reservation required, but if you do make one they are free so you may as well. The LNER (in fact most UK mainline routes) thing is down to incorrect information somewhere in the system. These journeys are flagged up as reservation required when this is 100% incorrect. There is no *requirement* to reserve a seat anywhere in the UK other than on the sleeper services.