[HappyHippy]

If the British Lop can farrow and rear this number of pigs, why is it a rare breed?

These one's are crossed with Hampshire - they're more prone to having big litters being more of a commercial breed (not in UK, but in USA)
I think maybe folks think because Lop's are a pink pig that there must be hundreds of them about, so choose something else assuming that all pink pigs in piggeries must be the same breed)
Middlewhites are the same - it's a shame really.
The pink breeds are a wee bit more susceptible to sunburn etc, and I don't think they're seen as hardy breeds for that reason (maybe another reason folks choose something else?)
But Lill's girl is so gentle that I can't understand why more folks would keep them - infact I told her I'd buy some if she ever bred them pure 
Karen 

[robert waddell]

the fertility comes of the mothers side     if she does not realease the eggs they are not going to get fertilized and as i said it is a different boar not that that made much difference with the pure Hampshire's they were smallish litters but there mother has big litters

[Rosemary]

Yes, I assumed the litter size was more the "responsibility" of the sow rather than the boar.

At least Middle Whites are spectacularly ugly :-) But I can see where the Lop and the Welsh might miss out in the eye-catching stakes. Shame really.