Come on - you are arguing for the sake of it, and your knowledge is dodgy.
Sciolistes has some astute points, and is a sensible post.
Banks are not passing base rate cuts on because no banks prices mortgage loans off base rate... they use the yield curve which reflects their cost of funding the loans. if the yield curve doesn't shift 3-4 years out which is the average duration of a UK mortgage product (before refinancing) then they do not change rates... simple as that. Base rate is irrelevent for pricing new mortgages
So what do you say to all those people on UK Base Rate tracker mortgages then? Admitedly, you won't find many new deals on such a basis, but UK base rate deals existed.
The pricing of the margin is based upon (a) competition and (b) cost of capital. A bank's cost of capital is determined by funding in interbank markets (LIBOR) and bond & equity financing. I have never seen pricing off government bond yields.
A loan is priced as Margin over an index (base rate, LIBOR, EURIBOR, or sometime CDSs of the borrowing company), and mandatory costs (compensation for BoE requirements). A mortgage is little different - all these are lumped into the "standard variable rate".