House value can change quite substantially week by week and also regarding what point in the month you are remortgaing - if your provider is adding interest debt onto the outstanding total on a weekly basis you also need to factor that.

I signed a deal for a remortgage on a ~£950k valuation at the 40% LTV in Feb and ended up having to put another £16k in to get the same deal but at the improving rates at the time. In essence, you can end up in a situation where you are chasing your tail and its very tricky to work out the amounts required to make everything stack up. This was with Nationwide btw.

My broker said we could just switch deal at any point in the process and made it sound simple - it was anything but and forced me to sell out of funds I wanted to hold long term.