Reddit reviews Writers' & Artists' Yearbook 2019 (Writers' and Artists')

You could do worse than picking up a copy of this. I haven't read a copy in a good few years, but it used to have a pretty exhaustive list of literary agents included - many of whom represent screenwriters.

In the meantime, have you looked into local funding agencies? Creative England are a good jumping off point for seeking out things like development funding or networking - there used to be more localised regional screen agencies, but most have now been folded into CE.