just for completeness, I thought I'd provide the link for the latest best practice guidance from OTA2
From the Ofcom website, OTA2 is:
"the Office of the Telecommunications Adjudicator is an independent organisation tasked by Ofcom to oversee co-operation between communications providers and enable a competitive environment in the telecommunications sector. OTA2 publishes best practice guides including the hosting of industry process documentation (e.g. processes for establishing a portability service, handling orders for number portability and associated service levels) for geographic and non-geographic number porting."
below is their best practice guide, including best practice for porting landline numbers:
https://www.offta.org.uk/best-practice-guide/
so, if you've got any issues with number porting, just read and digest all those documents, and you'll be absolutely fine
/sarcasm (if required)