A tracker mortgage is basically a kind of mortgage which mirrors the Bank of England's base rate. Therefore, if said rate goes up, your mortgage rate will go up. If the Bank's rate goes down, so will your mortgage rate. This type of mortgage is called a "variable rate mortgage" and is very popular for being cheaper than the rest. The best tracker mortgage rates are not hard to find. Since this is an affordable mortgage, the best tracker mortgage rates can be found anywhere.
Do not forget, however, that the best tracker mortgage rates will follow the Bank of England's base rate but will increase it a bit. After all, in order to provide you with the best tracker mortgage rates, lenders have to make some profit out of it.