Please do not respond to this post only to tell me to do a search. Thank you!

To those not wishing to be gratuitous: I am thinking of an MR to LHR, LGW or MAN, on which I could upgrade with VIP's. I have read in some posts about a luxury tax which is imposed on these upgrades. Would someone who is knowledgable of this mind clarifying the situation for me? I should be ever so obliged. Thank you.