On Sat, 3 Feb 2018 19:57:51 +0000, Raymond Brown <[log in to unmask]> wrote:

>Tho [ð] will certainly occurred and did also occur in the earliest
>French as a lenition of Latin [d] between vowels, there is no way in
>Britainese that [θ] would have developed from Vulgar Latin so at the
>moment Britainese has only [ð].

Not even secondary cluster developments?  With Latin on the mind I'm reminded of its development *sr > [θr] > [fr] with a transient [θ] -- I'm of course not suggesting that exactly that would therefore be supported in Britainese, but perhaps there's something like it.  You haven't discussed many small secondary developments in Britainese at all (yet?).

>There is no doubt that the Britainese scribes would have used the graphy
>_th_ already known in Latin for this sound.  But is borrowing it in loan
>words from Germanic enough for this sound to become part of Britainese?
>  If so, what other loan words?

It could also be a late introduction to Britainese, e.g. when Greek classical loans start getting taken up.  (For much the same reasons: a language with native /ð/ and no /θ/, but a voicing contrast in fricatives otherwise, would probably be amenable to filling the gap when it finds a donor with lots of /θ/.)