As many as nine out of 10 staff members at Channel 4 who are being asked to move to the British broadcaster’s new national headquarters and other satellite offices outside London are expected to quit rather than relocate this fall, chief executive Alex Mahon said Tuesday. That would mean the loss of about 270 employees – whose jobs the channel has already begun trying to fill – out of an overall payroll of 800.

The broadcaster is moving several of its operations from London in October, predominantly to a new national headquarters in Leeds, in northern England, but also to new hubs opening in Bristol in the west and Glasgow in Scotland. Channel 4 has committed to spending half of its programming budget outside of London, the traditional base for U.K. producers.

“There are about 300 Channel 4 jobs moving outside London – and about 3,000 jobs we expect to support in the wider creative community,” Mahon told a British parliamentary committee Tuesday. “We have always expected that, of the people that are offered for their role to move to elsewhere, we would have a fairly high degree of people who would not be able to take that up offer, or wouldn’t choose to.”

She said that when fellow pubcaster the BBC moved some of its operations to Salford, also in northern England, the attrition rate was 70%. Channel 4 has sought advice from the BBC, she said.

“We have always expected to be in the 70-90% range….So we are making redundancies and re-hiring in the nations and regions,” Mahon said, referring to Scotland, Wales and other parts of the U.K.

Generous severance packages and associated costs with the staff departures are likely to hit about £50 million ($62 million). As few as 30 staff could end up relocating outside London. Mahon said the upheaval had pros and cons. “The cons are a loss of experienced staff within Channel 4 and a loss of colleagues we wouldn’t want to depart,” she said. “The pros are we are able to hire people already based in the nations and regions.”

Mahon said several unspecified senior staffers whom she had expected to depart are staying. She herself will remain based in London, but will spend 20-25% of her time in Leeds, Bristol and Glasgow. “I have an expectation on all my executives to travel there, too,” she said.

Channel 4’s moves will coincide with the U.K. leaving the European Union, assuming the latest Brexit date of October 31 holds. ITV and others have warned of a Brexit shock for the TV ad market, which Mahon acknowledged, but she said that it would not affect the relocation. It could, however, have a wider-reaching impact on Channel 4. “If there was a massive knock to the advertising market, we’ve got to look at all cost lines in the business,” she said.