Current mortgage rates assume a reduction of 0.25% in the Bank of England rates in the last quarter of this year. This is widely assumed to be likely and lenders buy interest rate swaps, which in turn are priced based on assumptions about future rates. As those assumptions change, so do the prices for these swaps and in turn the mortgage rates you are offered for fixed deals.
Simply put, should the likelihood of a BoE base rate drop fall, then mortgage interest rates for new products will increase.
If this starts to become more likely then you are strongly advised to secure a deal now as we might see another uptick in rates as this develops.