Did you get a confirmation email for the 'collect from home' booking? If not then it sounds like it wasn't booked, for whatever reason.
Also check by going to your orders page, click 'View tracking' for that order and it should say "Collection Request Succesfully booked for item", and if that's not there either then it's not booked.
You can still book collection yourself through the Royal Mail website. That would mean not using ebay's postage. If you do that you mark dispatched by opening the original Simple Delivery label (Get prepaid label > Get prepaid label and QR code). It will remain unused and the cost will be automatically refunded to the buyer in 30 days (assuming they paid for it).
The option to change tracking number has been removed. You can message your buyer with the tracking details. If necessary you would be able to add the new number to an "Item not received" case if one was opened. Payment may be held for 14 days.
@pading_56