My british passport just expired. I hadn't been paying attention. Thing is, I cna't afford to renew it right now, so can I just renew later in the year. My 90 day window for filing my I-751 starts at the end of december. Does it need to be done before then?

Simon

Not a problem, you do not need an un-expired passport to lift conditions.

For the purpose of removal of conditions, you don't need a current passport. You don't even need your old passport, unless you're using it as evidence to support the bona fides of your marriage (for example, if stamps in your passport and your spouse's passport indicate the two of you have travelled together).

If you want to travel, of course, then you'll need a current passport, so it's probably a good idea to renew it as soon as practical at the closest british consulate.