To upgrade WTP to CW with avios you require 0.5x the economy avios, so yes that would be 35,000 each return for LHR-SIN.
Any published WTP fare can be upgraded with avios (so any that you purchase on BA.com).
To upgrade there must be CW award availability on your flight which is where you will likely have difficulty. These are often taken when they are released 354 days in advance. During a sale there may be no award seats available when you want to travel, so although you may get a good price for WTP you may not be able to upgrade. Award seats may be released at any time or close to departure, but there is no guarantee this will happen.
Currently it looks like there are 2 days with 2 CW award seats for LHR-SIN, and 3 days with 2 CW award seats for SIN-LHR.
If you have some time you may want to look at ex-EU CW fares.