This is something that will depend on your dates etc etc, however if you are flying on off-peak dates, then you will need 24,000 Avios per person each way (if only upgrading the LON-NYC flights). Therefore if you are only upgrading the long legs, it would be a total of 96,000 Avios for the two of you both ways. It looks like you would have to buy the 70,000 Avios package (i.e. 70,000 Avios + 35,000 bonus) to get enough for your upgrades which would be £1,135 worth of Avios. Then there would be a taxes/fees/charges increase of approx £200 per person total. That brings you to spending about £1,535 to upgrade the two of you. Whilst this is not going to be the worst use of money, there is actually quite wide availability to travel MAN-LON-NYC-LON-MAN for about £1,500 per person.

In conclusion, it depends on how much you spent on your current WT+ tickets, if they are on dates when there are cheap MAN-NYC tickets available then upgrading using cash (for the fare difference plus any upgrade fee) will probably be the better option as it seems that cheap WT+ tickets are about £750 and cheap CW tickets are about £1,500 per person meaning that the fare difference is £750 per person or £1,500 in total, the same as you would spend on buying Avios and taxes/fees/charges etc.

Also, if you upgrade with cash then you will earn the TPs and also Avios for being in CW.

N.B. I see here: https://www.flyertalk.com/forum/32292115-post11.html that you said you booked with BA Holidays, this normally means that there are no upgrade fees, only fare differences. With BA Holidays it is always best to call them up and ask how much it would be to upgrade with cash.