Bad luck

Your quote came back in on the same day as
eded2020. That was also a higher price than desktop, although nowhere near as high as yours.
We've seen a few desktop quotes of £5,100 or £5,200 recently. Even when the surveyor said it's a straightforward installation, the final price came out at £9K - or in your case much worse.
I think Openreach should stop giving out £5K desktop quotes when there's little chance of the final price matching that.