I think it is worth bearing in mind that there are very frequent CW sales at the moment. A few weeks ago you could have bought CW returns to New York for a very wide range of dates for £1100 (and briefly for £1050, I think). There will probably be a similar sale coming along again in a month or so. With prices at that level, what you're proposing isn't a very good use of miles unless it is a trip to New York you really have to book now (it doesn't sound like it is) while prices are £1800.

MFUs to the West Coast represent significantly better value; I believe CW returns to LAX have been around £1600 in sales this year. Even better is CPT, which costs the same miles as going to the U.S. and where the saving is substantially greater; availability tends to really come and go and sometimes can be very poor, but today seems to be pretty good for travel around November (the first month I checked) which is a lovely time to go.