I have used it to pay a large foreign tax bill, so not quite a house. If the amount is GBP£30000 (might have changed now), you can arrange a longer pay-in time (normally it is 24 hours) and they will let you have the cash sit in your account for up to a week and then find the right rate.
Later: Appears they have lowered the threshold to £20000 now (see https://transferwise.com/help/articles/2961856/tips-for-gett...)