As Z-M states, in any situation where a P45 isn't available (irrespective of the reason), employers are now obliged to use the 'Starter Checklist':
https://www.gov.uk/government/publications/paye-starter-checklist
P46 forms have been abolished.
Your new employer will probably initially allocate you to a temporary tax code, such as '0T', resulting in you having 20% of your pay deducted in Income Tax. However you'll get that back as soon as HMRC has advised the employer of the correct code to use.