Ok - nice dilemna
I haven't flown Virgin PE, but - on what I know - would book it over BA WT+. The new Virgin PE seat (if you get a plane with them fitted) is better than the WT+ seat.
PE gets a seperate check-in area, priority baggage delivery, a drink (champagne?) before take-off and different meals from economy (served on proper china). You may also get the opportunity to pay extra and use the Virgin ClubHouse Lounge at LGW.
WT+ doesn't always have a seperate check-in area, has no priority baggage delivery, has no drinks before take-off and has the same meal as WT (economy). Also there is no way to buy your way into the BA lounge (although you could buy your way into another airline lounge).
From what I know, PE is a much better all round product than WT+
Have flown UC and CW quite a few times. UC has a better seat for sleeping in (although BA's seat is better for relaxing in - more reclining options) and Virgin's Lounge is better than BA's (although BA's is not bad). BA tend to have more professional and consistent service/crew in CW (I have had both outstanding and dreadful service in UC) and BA's food tends to be better than Virgin's (BA don't have freedom dining like VS, but VS's UC food is getting some incredibly bad reviews at the moment

).
UC is trendier and more gimicky and the seats are the best business class beds in the sky (well, along with Air New Zealand - who use identical seats). BA tend to have a more consistent and professional product. Both are good in my opinion.
UC cannot compare to a "proper" First Class product (like BA's, AA's or United's First Class) - proper First Class is absolutely amazing

. Unfortunately it is ridiculously expensive as well - so I will have to wait for either the upgrade fairy to visit again or a lottery win.
Alice