Bridging loans do exist if you want to buy another house before yours is sold but I don't know the details.
When we bought our house on a buy to let, the bank were not interested in the value of the house, they wanted a statement from a local agency confirming the rental value of our project. They then loaned up to 90% of that projected income, including a huge amount for 'travaux'. We did have to provide quotes for said work, then as Kate said, they released that amount against receipts from wherever.
What may interest people is that we had the chance to defer the repayments for a year before we started paying back although apparently if we had used all the money before then we would have had to start the repayments immediately.
I do think it helps if you know the bank manager, not socially but we had been customers for a few years before we applied.