It should be possible to port Virgin to BT , in the same way it’s possible to port BT to VM, in fact the original VM ‘exchanges’ were pretty much the same as BT’s estate of System X and System Y, so technically if BT export, then so can VM, as the OP mentions’fibre’, if that’s FTTP , it’s possible that the location of the head end for the FTTP is what’s causing the issue as it may not be located at the end users local exchange, there was a problem initially with some BT customers having issues when taking FTTP and trying to keep the existing number but delivered over the ‘fibre’ rather than copper.
The very first (OFTEL) porting arrangements had to be reciprocal, back in the 90’s, the cable industry as it was , knew not being able to keep the phone number may hinder take up of their services, hence the regulator mandate to have number portability, but the regulators knew it would look bad if ‘BT’ were unable to ever win a cable customer ( hardly fair competition) hence the reciprocal nature of the arrangement ...in fact there were some ( never used ) but complicated rules to number port...if ‘BT’ had a exchange with a 10,000 mult. (number range) , for example 450000-459999 and the local cable company was especially successful in winning customers and signed up more than 50% , then it was deemed unfair for BT to actually have the expense of that number range but to but be exporting more numbers within that range than they had themselves, so the number range would have to be re allocated to the cable company and the remaining BT customers in the range would be ported from cable to BT .
VM exchange engineer staff were TUPE’d to ‘BT’ many years ago and ‘BT’ actually maintain VM’s switch estate ( although it’s possible VM are moving to an IP switching and the arrangement is out of date ) but it certainly was the case a few years ago.
