Benjamin (UK) interview questions

I was interviewed for this position through Microsoft Teams, with the CFO and HR manager. After an hour and a half I was offered a job in the spot (which I thought was a little unusual) and was immediately sent through all the documents to sign. This I did, but having lived abroad in the past, the company requested that I get a police certificate from the country I had previously lived in. Under the assumption that I had secured the job (I had signed the contract and received an enhanced DBS through the company), I spent £900 on a flight ticket to travel back to collect the police clearance. But ... I was suddenly told that I needed a further interview, after which I was informed (without explanation) that the company was no longer interested in hiring me, and was cancelling our contract. I found the entire process to be unprofessional and amateur, not to mention extremely costly.