SVR will VERY rarely be the same as the BOE rate.
In the old days, this would hold true. Banks only lent money which they got from depositors.
However, very few people got mortgages back then, because very few people saved. No money from savers, no money for borrowers.
This all changed though, and a typical bank is funded by 3 means. Savers (Retail), borrowing gilts, bonds, covered bonds etc (Wholesale funding) and Securitisation.
Therefore, the banks money funding costs have VERY little, if at all, to do with the BOE interest rate, or even the current LIBOR rate. (rate which banks lend to each other).
Why? When a bank borrows money, either wholesale or retail, its usually a variable rate. So if Bank of Craig51 went and borrowed £1bn from the money markets, it would be based at say 4% variable. However, for Bank of craig51 thats no good. I cant work out what my projected costs will be, or what rate to offer my customers, as my price of funding could change at ay moment. So today it might be 4%, but tomorrow it might be 6%. Thats bad because if I only get 5% from my customers, I make a loss.
So, banks then swap these variable rate funding vechiles for fixed rate vechiles. This might cost them, but then they know for a fixed rate of time, that slice of funding is fixed, so can then work out what rate to offer.
You may be on SVR, but the bank still needs to fund that.
Of course none of this applys with you have a true tracker, where some people are paying nothing right now if they have an interest only mortgage.
I could get a deal with C & G for 3.00% for 3 years with no fees 
