As for all discussions about ticket prices, if it seems like a price you're willing to pay at the time, then just do it! Prices can rise and fall, BA could have a sale after Christmas (in fact they often do) and the price might reduce a little further. As long as you are more than about one month away from your date of departure, then you should still be able to get the lowest price tickets. This starts to change from about 21 days before departure.

NYC to London benefits from lots of flights and some competition which is a good thing for you, so you're unlikely to end up in a situation where the costs for particular dates you want increase significantly in January. That said, of course, April can be a holiday month and many people might think that $900 is a reasonable price for such a ticket. It's your call in the end!