Out of interest I did a search on the IGI on familysearch.com. This searches old birth and marriage records generally from the 1700s-1800s in the UK. Using the common surname
Smith, I get 231
Lillias, 184
Lilias and 2
Lileas. Using another common surname
Campbell, I get 228
Lillias, 121
Lilias and 0
Lileas. So I stand corrected,
Lillias with 2 l's is the most common.
There is also 1 Lyleas
Campbell, which might reflect a different pronunciation but this record is from 1654 when spelling was even less consistent so I don't think you can draw anything from it.