Top up loans are generally separate to the exisiting mortgage as they are taken out after the original mortgage. When the lender offers the same rate for top ups as for standard mortgages (not in your case!), it means that although you are making two separate repayments the overall repayment would be the same as if it was just one loan.
However, if you were to remortgage you would have one loan, one monthly payment and one interest rate, and probably a better one than your current lender is offering you