Ive been looking too over the last few weeks and BA PE is around £500 cheaper and economy around £250. It pains me to say it but BA have the cheaper flights to the U.S at the moment. Holding in the hope of a sale or reduction to stay loyal but time running out to book.
Likely VS have sold out of the cheaper fare buckets. BA take A380s to LAX so with greater capacity they are able to sell more seats at cheaper prices. I wouldn't say it's the norm or VS would be out of business.
I had the same situation a couple months ago. BA MUCH cheaper than virgin. I waited it out a bit as id prefer not to go with BA and luckily virgins price dropped lower than BA so I went ahead and booked. I booked 6 months in advance...closer to the date though it may be a little risky to wait it out and hope