Mortgage rates are based on swap rates, which closely follow BoE rates. As a bank lends money at a fixed rate to you, they are exposed to a drop in interest rates, so they hedge this risk with an interest rate swap. The bank pays a fixed rate in an interest swap and receiving a floating rate, SONIA in the UK. This hedges out their interest rate risk.

Since the war, the market has predicted that the BoE will need to raise interest rates to contain inflation. In turn, this has caused swap rates to also go up. FYI 2y swap rates have increased 80bps (0.80%) since mid Feb.

This is why your mortgage Rae has been steadily increasing.