Personally for me, at £615 I would do it, but as everyone else has mentioned, it depends on how much you have and are willing/able to spend.
WTP is all well and fine though and is my 'go to' mode of travel, when flying from London to USA. The seats are a bit bigger & wider than WT. The food service you get is better than WT too, but with CW you do get a seat that does convert into a bed.
Then the questions becomes, are you travelling during the day, or on an over night flight? - For me, when flying the redeye I'd much rather have a full flat bed than sitting upright, (or slightly reclined at a jaunty angle).
I had a similar conundrum last week, about an upgrade from WTP to F (for £1300) from LHR to HKG (Given that was a 12/13 hour flight, decided to go for it)