Order of buying process…London

We are in the process of selling our house, standard 3-bed semi in Wembley. The buyer had their mortgage & survey but refused to start any conveyancing until the mortgage was confirmed. It’s been years since I moved/sold before and I’m moving in with my elderly parents so I’m not part of a chain to remember what the steps are. Ist what they’re saying reasonable? Shouldn’t it all happen in parallel so it’s not all woefully slow. It feels like they’re trying to slow the process down.